Does Lowering BAC Limit To 0.05% Actually Save Lives?

We at Twenty-One Debunked have long supported lowering the legal BAC limit for DUI/DWI to 0.05%, with nuance and graduated penalties, based on what we thought was rock-solid research supporting such a move from the current 0.08% limit in most of the USA.  However, a pair of more recent British studies casts serious doubt on whether such a move will actually save any lives at all by itself.

The study, in the prestigious medical journal The Lancet of all places, looks at a sort of natural experiment where Scotland lowered their BAC limit from 0.08% to 0.05% in 2014, while England and Wales kept it at 0.08% throughout.  The results?  Scotland saw no significant absolute difference in weekly traffic crashes after the change compared with before, and even saw a statistically significant 7% increase in crashes relative to England and Wales.  Similar results were found when looking only at serious, fatal, or single-vehicle nighttime crashes, hence the results were robust as well.  And all of this was after adjustment for several possible confounders.  The study also found that, contrary to what some might predict, no overall change in per capita alcohol consumption in Scotland either, and only a 0.7% decrease in on-premise alcohol consumption.  Can you say, "Whoops!"?

(And no, the study was NOT funded by the alcohol industry or any of its front groups.  Sorry.)

Thus, it seems that while lowering the BAC limit from 0.15% to 0.10% and then to 0.08% long ago probably did save a significant number of lives (though the exact number is debatable), further lowering it from 0.08% to 0.05% has very diminishing returns at best.  And it can perhaps even backfire at worst. The Australian success story was far more likely due to very tough enforcement of DUI laws in general (including, but not limited to, their legendary random breath testing (RBT) program), while the Continental European success story was likely due to the same, plus world-class public transportation as well.

So why are the above results so lackluster then?  Well, as Darren Grant noted in his famous study debunking the effectiveness of zero tolerance BAC laws for people under 21, it should really come as no surprise to an economist, who is used to thinking on the margin.  While driving impairment indeed begins long before reaching 0.08% or even 0.05%, it rises exponentially with each drink.  Thus, higher BACs are exponentially more dangerous than lower ones.  And the decision to have one's next drink based on trying to stay within the legal limit, even if a lower limit had the same deterrent effect at that limit, would have exponentially less effect that the decision not to cross a higher threshold.  (If anything, it's even MORE exponential for younger drivers, not less.)  And apparently once when the limit goes down to 0.05% or lower, any additional lifesaving effect over and above lowering the limit to 0.08% simply gets lost in the statistical noise.  That is especially true if the penalty is the same for crossing a lower threshold, since there is literally no marginal deterrence effect against each additional drink beyond that threshold.

(Grant also did a sort of sequel to that study in 2016, again drawing similar conclusions.)

We already know that the vast, vast majority of DUI casualties are concentrated among extremely high BAC drivers, usually 0.15%+ (average 0.16%) and often ones who do so repeatedly and frequently.  These drunk drivers are sometimes called "hardcore" drunk drivers, and are the most resistant to changing their behavior.  And most of those are in fact alcoholics to one degree or another.  They would, of course, laugh at the idea of a BAC limit being lowered to 0.05%, or at the very least, wouldn't exactly agonize over it.  This is all common knowledge, and not at all controversial, except of course among MADD and similar zealots.

Thus, if we as a society decide to set the BAC limit at 0.05%, or indeed any number below 0.08%, there should be steeply graduated penalties, with 0.05% (or 0.02% for young or novice drivers) being a mere traffic violation with a modest fine and short-term license suspension, and no criminal record, 0.08-0.10% being a misdemeanor with a steeper fine and longer license suspension or revocation, 0.10-0.15% being misdemeanor with an even steeper fine and even longer license revocation and mandatory jail time, and 0.15%+ being a felony with permanent or semi-permanent license revocation, very steep fines, vehicle forfeiture, and a stiff prison sentence.  Repeat offenses of 0.08% or higher would carry the same penalties as a 0.15%+ BAC, as would driving above 0.08% with a child under 16 in the vehicle.

We also know that swiftness and certainty of punishment, or the perception of such, is a far greater deterrent than severity.  Thus, automatic administrative penalties, separate from any criminal penalties, are found to be more effective that any criminal laws on the books, most notably in the Canadian provinces of British Columbia and Alberta.

Of course, we would be remiss if we did not discuss the stunning success of special DUI/DWI courts.  These are diversion programs that allow DUI offenders to plead guilty and complete coerced treatment and enforced abstinence from alcohol instead of jail or other penalties.  And they work quite well, with far lower recidivism rates than for those who do not enter such programs.  There are two types of DUI offenders these days:  1) relative normies who simply make dumb decisions and can be "scared straight", and 2) hardcore drunk drivers.  And DUI courts actually get to the root of the problem for the latter.

But first, we need to get these ticking time bombs off of the roads before they kill or maim innocent people.  Australian-style RBT is one blunt way to do it, but that would not likely pass constitutional muster in the USA. And even then, the long-run effects of Australia's RBT were not very different than those of American-style sobriety checkpoints when studied in the 1980s.  Tougher and sustained enforcement in general really seems to be the key.  And that can of course be done more effectively and cost-effectively with roving and saturation patrols, that look for signs of actually impaired drivers on the roads.  Especially in the many states that do not allow sobriety checkpoints to take place, they HAVE to rely on DUI patrols instead.

(Twenty-One Debunked recently came up with an ingenious idea to turn roving and saturation police patrols against DUI into a COPS-like reality TV show called "Operation Rovin' Eyes", complete with ride-alongs for community members as well.)

Seasoned drunk drivers know how to avoid the checkpoints with ease.  But with roving and saturation patrols, they will soon learn (either the easy way, or the hard way) that you can run, but you can't hide.  "Roving eyes...are watching YOU!" would be a good slogan to publicize the program.  The "fish in a barrel" method, that is, parking a police car outside bars, clubs, or parties and catching would-be drunk drivers before they get on the road, would also be a great complement to such patrols as well.

And while we're at it, let's get all of the garden-variety reckless, negligent, and distracted drivers off the road as well.  The same patrols will of course get them too.

It's time to stop tilting at proverbial windmills, and finish the job for good.  So what are we waiting for?

P.S.  The drinking age in the UK is 18, in case you didn't already know.  Kinda like most of the world.

UPDATE:  What about weed?  Unlike with alcohol, Twenty-One Debunked does not support any per se limits for cannabis, only rational prima facie ones that are rebuttable, similar to what Colorado has.  The complex pharmacokinetics and lack of correlation between blood THC levels and actual impairment renders any per se legal threshold to be inherently unscientific, and there is still zero evidence that such per se laws save any lives or improve public safety at all.  While cannabis can indeed impair driving, and a fortiori when combined with alcohol, it is best to focus on those drivers that are actually impaired, rather than to snare perfectly sober drivers in the dragnet due to mere trace amounts of THC in one's system.

Japan Actually Encouraging Young Adults To Drink More Alcohol

Concerned about falling alcohol tax revenue, as well company profits, the government of Japan is now literally encouraging young adults to drink more alcohol.  Yes, you read that right.  They are even going so far as to hold a contest.  Apparently, alcohol consumption among young people has been falling since the 1990s, and even more so recently, with tax and business revenues falling along with it.

But most surprisingly of all, they are still NOT planning on lowering their drinking age to 18 to match their new age of majority.  They lowered their general age of majority earlier this year from 20 to 18, but still kept their smoking and drinking ages at 20, because reasons.  Or something.  Yes, really.

Stranger than fiction indeed!  And not nearly enough (facepalm) in the world to cover this!

Legalization Of Cannabis Still Not A Disaster After All

One of the zombie lies that cannabis prohibitionists luuurrrve to shill is that legalization would inevitably result in mass carnage on the highways (if not also off of the highways as well) from stoned driving.  These fearmongers can be every bit as hysterical as MADD is infamous for being in regards to alcohol.  Studies to date on the matter have been mixed, but have been generally pointing in the direction of it being largely a non-problem overall, at least compared to the counterfactual of not legalizing cannabis.  While it is true that cannabis can impair driving, it typically does so less than alcohol does, and regardless it does not necessarily follow that legalization would automatically result in more traffic casualties overall that would otherwise have occurred.

Enter the latest study by the Insurance Institute for Highway Safety (IIHS) using data from 2009-2019, the largest study yet on the matter.  While it purportedly found that recreational legalization was associated with a small increase in both fatal and nonfatal injury crashes overall, there is far less here than meets the eye.  For starters, there seemed to be no rhyme or reason to which states saw increases and which ones saw decreases (yes, some states actually saw decreases post-legalization) in such casualties, even after controlling for a whole host of potential confounders and including several comparison states (five legalization states and five non-legalization states from the same general region of the country). Such extreme heterogeneity strongly suggests that at least some, if not all, of the observed effect is spurious.  More importantly, legalization of recreational cannabis use itself appeared to be responsible for nearly all of the observed effect, while legalization of retail sales (which occurred later, often a year or more later) had no significant effect (actually, a small decrease on average, go figure).  You read that right--it is exactly backwards from what the standard prohibitionist narrative would claim, also suggesting a spurious effect.  And finally, in any case the size of the effect (5.8% for injury crashes and 4.1% for fatal crashes) on average is basically too weak to conclusively prove a causal link from a single study alone, but especially from a study with the aforementioned issues like this one.  

In other words, one can see that there is barely any signal in the noise, and that supposed signal may very well turn out to have been entirely noise all along.

So once again, legalization was NOT anywhere even remotely close to disaster after all.  And most interestingly of all, the study authors note that, “the estimated effect of marijuana legalization on crash rates is only slightly lower than the estimated effects of lowering the legal drinking age in the United States from 21 to 18.”  Which, as we at Twenty-One Debunked have noted time and again, are both nowhere near as large or scary as the prohibitionists and ageists want us to believe.  And an honest interpretation of the aforementioned study would actually put it far closer to the famous Miron and Tetelbaum (2009) study than any pro-21 study.

UPDATE:  Nevertheless, we skeptics do of course need to be careful not to give into the temptation to be "pseudocritics" here.  Blithely, casually, and reflexively mocking and dismissing any paper for its apparent statistical noise or seemingly puny effect size while ignoring nuance, details, or context (often without even reading the paper in question) is intellectually lazy at best, and intellectually dishonest at worst.  The above analysis, however, is NOT an example of this pitfall.  Pointing out specific concrete issues or inconsistencies that seriously weaken or vitiate the conclusions, while keeping it all in context is not an example of this fallacy.  Nor is pointing out, a fortiori in fact, the public policy implications or lack thereof (a second-order question that inherently goes beyond the science itself) that the prohibitionists so desperately wish for despite being a category error.  And those who doggedly cling to even the flimsiest cherry-picked results and tiniest effect sizes, to push a thoroughly illiberal agenda while ignoring an honest cost-benefit analysis in the name of a perverted and Hobbesian version of the precautionary principle, are guilty of a categorically far worse intellectual failing than even the silliest pseudocritics are.

When we see that cannabis legalization and non-legalization states in recent years, and MLDA 18, 19, 20, and 21 states in the 1970s and 1980s, are all more or less within error bounds of each other overall in terms of traffic casualties, at least in the long run, it makes far more sense to err on the side of liberty.

And finally, to say that small effect sizes, especially with wide confidence intervals or large heterogeneity, are weak correlations and thus should be approached with caution before jumping to conclusions is NOT the same as saying they should be automatically dismissed out of hand.  Those who argue otherwise don't seem to do nuance well, to put it mildly.

Cannabis Use Associated With DECREASED Covid Severity

Two and a half years after the COVID-19 pandemic began, our unofficial working theory was finally confirmed:  cannabis use was recently found in a new study to be associated with decreased Covid severity, contrary to what some pundits have claimed.  Remember, our own theory was that that was why states (such as Colorado, Massachusetts, Maine, Vermont, DC, Alaska, and the entire West Coast) and countries (Canada, Netherlands, and Uruguay) with at least de facto legalized recreational cannabis (as of January 2020) saw lower Covid-related death rates compared to places where cannabis was not yet (if ever) fully legal for recreational use by early 2020, and especially when compared to their neighbors.

Some exceptions are Michigan, Illinois, and Nevada, of course.  Those states did not do very well with Covid despite cannabis legalization in place by 2020, but the first two were latecomers with relatively delayed implementation of their legalization.  And the Netherlands, contrary to popular opinion, does not have full legalization, but rather a quasi-legalization scheme where, as the locals there would say, "the front door is legal but the back door is illegal".

But overall, it seems our theory panned out quite nicely.  Even when looking simply at cannabis use rates, rather than legalization, a similar pattern emerges overall.  Think the Southern US states vs Northern states, or Easten Europe vs Western Europe, or even Sweden vs. Denmark to some extent.

Game. Set. Match.  Any questions?

"No Safe Level" Junk Science, Again

Is there really "no safe level" of alcohol use before the arbitrary age of....40?  And do the risks always outweigh the benefits from the outset?  A new study, funded by the Bill and Melinda Gates Foundation, seems to claim that, just four years after the same "experts" claimed that there is no safe level for anyone of any age.  But again, as we already noted back then, the specious claim of "no safe level" is a classic red flag for manufactured junk science with a hidden or not-so-hidden agenda, dressed up in the guise of the precautionary principle.

First, the ontological criticism:  It is well-known that excessive alcohol consumption is harmful in so many ways, for all ages.  But if even (gasp!) radiation supposedly has a safe level for practical purposes, why on Earth can't that be true for alcohol as well?  Our own bodies actually produce small amounts of the stuff endogenously, after all.  (Ok, so they concede maybe a shot glass of beer (lol) per day might be safe.)

Second, there is of course reporting bias:  heavy drinkers tend to play down how much they drink in surveys, so the safe drinking thresholds may very well be higher than survey results suggest.  Who knows?  Also, they notably did not distinguish between different patterns of drinking either.  There is a world of difference between having two drinks every day, and having all 14 drinks (!) in one night per week, despite them both being the same average amount of alcohol per week.  (Not that Twenty-One Debunked actually encourages anyone to do either, by the way.)

Third, conveniently and arbitrarily walking back the original conclusions for the dominant age group in society, but not for younger folks, really smacks of special pleading and card-stacking fallacies.

Fourth, even if it were true, it does not follow that prohibition, age based or otherwise, is the solution.  I mean, since when did "give me liberty of give me death" become "give me absolute safety or else"?  Even from a strictly utilitarian perspective, people who sometimes choose pleasure over longevity to one degree or another are not necessarily irrational or even immoral.  Like it or not, the alcohol genie is out of the bottle, and banning it or unduly restricting it will only make it that much more dangerous by driving it underground, as we learned the hard way.

If the powers that be are so concerned about the "externalities" that even light to moderate drinkers supposedly impose on society, simply tax alcohol more then. Problem solved.  Otherwise, they really need to stay in their lane and leave people alone.

And finally, consider the source.  The Gates Foundation may want to look in the mirror before pointing the finger at others, and particularly look into the contents of the vials of their favorite experimental product du jour, if you catch my drift.

Oh, and notably, they did seem to distinguish over 21 versus under 21 either, so anyone who has the gall to try to use this specious study to somehow justify the 21 drinking age is really grasping at straws.

More Things Underage Drinkers Didn't Do

In the past, we would chronicle countless examples of drinkers OVER 21 behaving badly, as "Things Underage Drinkers Didn't Do".  But we have clearly been slacking lately.  Time to do it again:

In just the past few weeks or so in America:

An underage drinker did NOT get so incredibly wasted that she crashed her car into multiple barricades and then abandoned her six month old twin babies.  (Let me guess, she "misplaced" them.)

An underage drinker did NOT get another unrelated DUI just a few short months after a drunken (and cowardly) hit-and-run crash.

An underage drinker did NOT drunkenly crash her car into a utility pole and kill her 17 year old passenger despite being largely unscathed herself.

An underage drinker did NOT drunkenly crash his truck into and kill a beloved music teacher who just happened to be at the wrong place at the wrong time.

An underage drinker did NOT drunkenly run over and kill an innocent 13 year old girl walking down the grass shoulder of the street, and then speed away and casually keep on driving to Taco Bell as if nothing had happened.  (Another cowardly hit-and-run.)

An underage drinker did NOT pound five shots of tequila in a row at a Las Vegas casino, get behind the wheel, and then drunkenly crash into an innocent family's car, tragically killing the father and seriously maiming his 3 year old son.  (Also a hit-and-run.)

An underage drinker did NOT have the chutzpah to drive himself to court drunk (!) to fight charges for his third DUI.  Literally the definition of chutzpah!

Nor did an underage drinker have the chutzpah to drunkenly drive his SUV to the car dealership with a missing tire, obvious damage, and smoke to try to SELL the very same vehicle that he had just drunkenly crashed into the median several times after downing four Four Loko cans (aka "blackout in a can") and chasing that with a can of White Claw.  His baseball star father must be so proud of him! NOT.

The recent epidemic of unruly airplane passengers, most of whom are over 21, has been greatly fueled by alcohol.  (Fortunately it seems to be declining now, since nearly two-thirds of the misbehavior was linked to the federal in-flight mask mandate that is no longer in effect now, and also thanks to the new FAA zero tolerance policy for misbehavior while flying.)

Alcohol-related deaths spiked during the pandemic, mainly among people OVER 21.  America was already drowning in the bottom of a bottle, and the pandemic and lockdowns were like gasoline on the fire.

And finally, it was just recently revealed that Rudy Giuliani, 78, the questionable advisor to Trump who advised him to prematurely and falsely declare victory over Biden in the 2020 election, and thereby promulgating the Big Lie that incited the Capitol insurrection riot on January 6, 2021, was most likely drunk when he dispensed his questionable advice.

And that, my friends, is just the tip of the iceberg....

Dishonorable mentions outside the USA:

There was a 35 year old man in India who had meant to ritually sacrifice a goat, apparently got so wasted that he ended up inadvertently doing a HUMAN sacrifice of the person holding the goat.  Oops!

UPDATE:  But wait, there's more!

An underage drinker did NOT get in such a rage from being "mad at his girl" that he broke into the Dallas Museum of Art and wantonly destroyed $5 million worth of ancient art.  Just because, grrrr, he was pissed off! (No alcohol was mentioned in this case, but come on--when what the last time you heard of anyone of any age doing something like this sober?)

There's Nothing Progressive About Raising Gun Age To 21

With the latest mass shooting, of an elementary school no less, having a barely 18 year old perpetrator who purchased his AR-15 style semiautomatic rifle legally, there is now a renewed push from "progressives" to raise the age for purchasing at least some types of long guns from 18 to 21.  But there is really nothing progressive about doing so at all.

As long as the age of majority is 18, there is really no legitimate reason to selectively infringe upon the Second Amendment rights of 18-20 year olds.  Period.  It is both over-inclusive (most 18-20 year olds are not criminals, let alone murderers) and under-inclusive (the lion's share of mass shootings are committed by people over 21).  If anything, gender is a far bigger predictor of mass shootings than age, with men committing 96-99% of them, yet no one is calling for disarming all men.  And recently, a California law setting an age limit of 21 for semiautomatic rifles was struck down as unconstitutional.  But now Congress is trying to pass a federal law raising the age for at least some types of long guns to 21 (it is already 21 to purchase handguns under federal law).

It is indeed very important to keep guns from falling into the wrong hands, no argument there.  But there are far better (albeit imperfect) ways to do this:

• Require a license or permit to purchase handguns and some or all types of rifles going forward, regardless of age
• Require a psych evaluation before purchasing guns going forward, regardless of age
• Universal background checks
• Safe storage laws
• Red flag laws (truly a no-brainer)
• Waiting periods to buy guns
• "Treat bullets like Sudafed"
• Tax bullets (like Chris Rock recommended)
• Ban high-capacity magazines
• Reinstate the assault weapons ban that prevailed from 1994-2004

None of these commonsense measures actually violate the Second Amendment as written (the best part is where it says "well-regulated"), nor do they discriminate by age against young adults either.

Oh, and speaking of red flags, apparently the utterly deranged and disturbed mass murderer of Uvalde had so many red flags (including egregious ones like animal abuse, death threats, and rape threats) that he proudly waved them in front of everyone's face like a matador's cape in front of a bull.  And yet, no one did anything about it, for whatever reason, and he still got his AR-15 style rifle and lots of ammo very easily regardless.  Really makes you wonder!

Do Tobacco 21 Laws Really Work?

A recent study claims that Tobacco 21 laws work to reduce teen tobacco use.  But upon closer examination, there is less here than meets the eye.  While this new nationwide study controlled for a host of other variables and spanned from 2012-2019, it still leaves the reader with more questions than answers.  

For example, the supposed effect in survey data in grades 8, 10, and 12 was statistically significant for combustible cigarettes, particularly for grade 12, but NOT for vape products at any grade level, while for the Nielsen sales data the apparent effect was much larger for vape products than combustible cigarettes.  How exactly can this glaring contradiction be explained away?  Also, no significant effect was observed on heavy smoking, which intuitively should have had the largest effect size of all due to heavy smokers facing the greatest "hassle costs" when cigarettes become even somewhat more difficult to obtain.  And even for cigarette smoking in general, the absolute difference-in-differences was a few percentage points at most, for a substance whose popularity and prevalence was already declining among all ages long before the age limit was hiked to 21 (in contrast to vaping, which increased dramatically among young people during this period).

Furthermore, the study did not look beyond the first two years post-enactment of such laws, so any observed effects may very well be merely short-term effects (cf. Miron and Tetelbaum 2009 for the 21 drinking age and alcohol-related traffic fatalities).

Interestingly, the study authors do concede that at least some of the observed effect on people under 18 is a result of increased ID checking and increased perception of risk of of tobacco products.  Of course, we know that it is not really necessary to raise the age limit higher than 18 to achieve such effects.

Overall, this nationwide Tobacco 21 study (the second of its kind) was rather less impressive that an earlier nationwide study by different authors, for reasons that are not entirely clear.  But it is very likely a repeat of what we like to call "The Incredible Shrinking Effect Size", a phenomenon that we have seen before with studies of the 21 drinking age over time.  Which means that it was most likely a statistical mirage all along, at least in the long run.

Regardless of what sort of effects, or lack thereof, that Tobacco 21 laws supposedly had, we at Twenty-One Debunked nonetheless oppose such laws on principle 100%, just like we oppose the 21 drinking age and toking age as well.  Let America be America Again!

UPDATE:  A recent pro-21 article made a Freudian slip about what happened since Texas raised their smoking age to 21 in 2019:

"Since then, the use of tobacco by teens decreased from 21.8% in 2019 to 19.1% in 2021, according to the CDC and the American Lung Association."

That is NOT a particularly large difference in teen tobacco use (which includes both smoking and vaping), especially since teen vaping had already peaked by 2019 and teen smoking had already been plummeting for decades.  Technically, the latter data were from the 2020 Texas School Survey, not 2021, and the 2019 data were from the CDC YRBS, as each survey is done only every other year.  But either way, the point still stands.  In fact, when compared to the 2018 TSS data, the 2020 TSS data are almost identical.

2023 UPDATE:  The study in question has been recently revised, but not in a way that materially alters any of the above conclusions or criticisms.  In fact, reading between the lines, not only does the results look even less consistent and statistically significant than before, but now the effect size on survey results seems to be much larger for vaping than for smoking at all grade levels, implying that some vapers have likely switched to smoking.  That makes the Tobacco 21 policy a net public health loser.

2024 UPDATE:  A 2021 study of California, one of the first two states to implement a statewide Tobacco 21 law in 2016, showed weak and unimpressive numbers in a difference-in-differences analysis.  Womp womp.

Have A Safe And Happy Memorial Day Weekend

This coming Monday, May 30, is Memorial Day, often known as the unofficial first day of summer and National BBQ Day.  But let's remember what it really is--a day to honor all of the men and women of our armed forces who made the ultimate sacrifice for our country, past and present.  And that of course includes all of those who died serving our country before they were legally old enough to drink.  Let us all take a moment of silence to honor them.

As for Candy Lightner, the ageist turncoat founder of MADD who had the chutzpah and hubris to go on national TV in 2008 and publicly insult our troops, may her name and memory be forever blotted out. 

And as always, arrive alive, don't drink and drive.  It's just not worth it, period.  And it's very simple to prevent.  If you plan to drive, don't drink, and if you plan to drink, don't drive.  It's not rocket science.   Designate a sober driver, call a cab or rideshare, crash on the couch, or even walk if you have to.  Or don't drink--nobody's got a gun to your head.

Legalization Of Cannabis For Adults, Even With Age Limit Of 18, Does NOT Significantly Increase Teen Use

Lest anyone think that the feared massive post-legalization surge in teen cannabis use somehow NOT materializing in the USA (in legalization states) was somehow due to the age limit being set at 21, a recent study found that Canada also did not see a post-legalization surge in teen use either, and their age limit was 18 in Alberta and Quebec and 19 in other provinces at the time of the study (Quebec has unfortunately since raised it to 21 as of January 1, 2020, but the study ended with the 2018-2019 school year while it was still 18).  There was no statistically significant difference between cohorts of students before and after legalization in Canada.  And while Alberta had the highest reported rate of high school cannabis use of the handful of provinces studied, both before and after national legalization in October 2018, Quebec interestingly had the lowest.  And the other two provinces studied (Ontario and British Columbia) were somewhere in between.  

Note that modest short-term increases in occasional or experimental use, by perhaps a few percentage points, are actually to be expected, if only because they will likely be more honest in surveys now.

As for the specious claim that Uruguay (age limit 18) saw an increase in teen cannabis use after legalization in 2013, that claim was based on cherry-picking two arbitrary years:  2003 and 2014, without considering more recent trends.  And even in the Netherlands, there was no increase in teen cannabis use that could be unambiguously linked to their policy of quasi-legalization for adults over 18 since 1976 (note that it was originally 16, just like the drinking and tobacco smoking age once was, in most of the country until 1996).

Point is, just like with alcohol or tobacco, teens who are inclined to use cannabis will do so whether or not it is legal or not for them (or their slightly older near-peers) to do so.  And like alcohol (but unlike tobacco), most of those who experiment with it will not become regular users, let alone problem users or addicts.  And prohibition of either for adults, including young adults, clearly does NOT stop teens from using.

Thus, there is no good reason to set the age limit any higher than 18.  Period.

UPDATE:  Looks like Uruguay actually saw a decrease in cannabis use among youth under 18 following legalization.  For 18-21 year olds, there was a modest short-term increase in 2014, followed by a decrease.  Note that Uruguay set their cannabis age limit at 18.

And back in the States, guess what?  Contrary to the doomsayers, legalization still did not cause an increase in teen cannabis use as of 2023.