Caffeine

To paraphrase Eric Clapton's famous song:

If you wanna wake up, you gotta drink a cup 

CAFFEINE  

When your day is done, and you wanna ride on 

CAFFEINE   

She's all right, she's all right, she's all right...

CAFFEINE

Or, to paraphrase The Grateful Dead:

Drivin' that train, high on caffeine 

Casey Jones you better, watch your speed!

It's no coincidence that coffee (and/or tea, depending on the country) is the world's number-one beverage--and caffeine is thus the number-one drug as well.  And in fact it is the only major psychoactive substance that is both readily available everywhere and does not have an age limit, as it is officially considered a food rather than a drug.  And while caffeine does indeed have a dark side and can be harmful in excess (and/or for those who are unusually sensitive to it), and can also be addictive as well, the best studies show that in moderation, coffee/tea is relatively safe and even beneficial for most people.  In fact, it is a classic example of how a natural, botanical drug can be successfully tamed by society overall, even if some do abuse it and/or experience adverse effects.  At the same time, like any other substance, when caffeine is isolated/concentrated from its source, one can observe that it becomes riskier and more prone to abuse.  After all, "the dose makes the poison".

In fact, some studies find that moderate coffee drinkers live longer than non-coffee drinkers.

As for the theoretical risks of aggression and violence resulting from caffeine, as well as it supposedly being a "gateway drug" for teens, the jury is still out on both counts for the time being.  Correlations have been found in some studies, but we all know that correlation alone does not prove causation.  If these are in fact true, they would most likely be mainly the case for people under 15, though no age group would be completely immune either given high enough doses and/or enough of an innate and idiosyncratic vulnerability.  (Just Google "caffeine psychosis".)

Recently, there has been a push to set an age limit of 18 for energy drinks, and Twenty-One Debunked opposes any effort to do so.  If they are really so concerned about it, they should have better labeling requirements, cap the caffeine content in energy drinks, and/or tax the hell out of such beverages.  There is no hard scientific evidence that caffeine is any more hazardous or harmful for 15-17 year olds than for people over 18, though for people under 15 more caution is warranted.  Regardless, it would most likely be counterproductive to set an age limit of 18 for energy drinks (if they must, no higher than 16) that don't contain alcohol or any other stimulants or other psychoactive substances stronger than caffeine.

And of course, perish any thought of setting any legal age limits at all for coffee or tea!  A modest excise tax perhaps (though for tea in particular, the Founders would most likely be spinning in their graves!), but nothing more stringent than that is warranted.

Above all, let's NOT create or stoke a moral panic about energy drinks or caffeine in general, as that will backfire mightily.  No "Java Madness" campaigns or anything like that.  The BBC article by Kate Fox was supposed to be a warning, not an instruction manual!

But do be sure to use caution at any age if currently pregnant or attempting to become pregnant.

And finally, for those who wish to quit caffeine, or even just a temporary break to reset one's tolerance, see here for a method that is far less brutal than cold turkey, and yet even easier than gradual weaning.  According to John Fawkes, it can work in as little as four days, and may also work for addictions to other stimulants or dopamine-producing drugs as well.  Also see here as well for further information. 

P.S.  Caffeine is not the only psychoactive and bioactive component found in coffee.  There are others as well, that can also be found in decaf as well as full-strength coffee.