(Honestly I think the legal age for any drug should be 21 all 'round...You know, at roughly the point where it doesn't get to play havoc upon developing brains.)
Oh, that's a horrible idea. You know what's great about letting kids drink? Their parents are still around. So you start by sharing a beer or two that somebody smuggled away from his parents, then you start having a couple beers a night, but no liquor, since that's hard to get... Usually one of your friends has permissive parents, so you'll get the stuff before you're legally allowed to buy it yourself. But whatever you try, a responsible adult is never that far away, and you're usually in a safe environment, so nothing really bad can happen. The absolute worst would be puking your guts out, but your parents' wrath will keep a check on that, too.
And so by the time you're on your own, living in a big town, buying your own groceries, studying or whatever, alcohol is just another thing. It's not a taboo to break, and getting shitfaced is not a way to prove your independence from your parents - it's just a way to embarass yourself in front of your friends, showing them you can't handle your liquor.
TL;DR: Alcohol is a cultural thing, so people need to learn how to deal with it. Being introduced to it slowly and under a degree of supervision is preferrable to ostrich tactics and college binge drinking.