@victorinox: that's because people don't have any experience with alcohol, and vastly over-estimate the amount they "need" to drink.
I had my first drink (wine) with my parents at 13 or so. It went straight to my head, so when I got to be about 16 and was going to parties I knew to take it easy. Despite my rep as one of the biggest drinkers around these parts, and having been drinking for a decade and a half, I can count on the fingers of one hand the number of times I've vomited from too much drinking. I know my limit, having learned it young.
Drinking ages don't set limits. They represent challenges. Take away the rule, you take away the challenge.
There's a difference between a drinker, an alcoholic and a douchebag. One drinks and likes it, one drinks because he is compelled and one drinks to prove something. I'll let you figure out which is which.