A recent rat study has been done that appears to justify the 21 drinking age at first glance. But here's why it really doesn't:
1) First of all, the study was done on rats, and rats are NOT people.
2) There is no rat equivalent for 18-24 year old humans, as the "adolescence" in all rats essentially overlaps with infancy and does NOT overlap with adulthood.
3) The human brain actually continues to develop well into the 30s and 40s, so 21 is an arbitrary age limit.
4) Human brain development before 18, and especially before 15 or so, is qualitatively different and occurs on a much more fundamental level than that which occurs afterwards.
That's not to say that excessive drinking isn't harmful. It is, at any age in fact, and especially so before 18. But there is really no conclusive scientific evidence that drinking at 18 is any more harmful than at 21.
Besides, if the results of the numerous rat studies of the past actually did translate to humans in like fashion, then Canada, Australia, and Europe would all be nations of brain-damaged alcoholic felons!