Perhapse 21 may seem a bit high to drink in the US, but what are the rates of alcoholism in countries with lower drinking ages? We changed the drinking age to 21 back in the mid-80's because we had a lot of problems with people who couldnt make good choices when drinking - drinking and driving, for instance.
Now thats not restricted to college age drinkers to be sure, but it forces some limits on people, many of whom are away from any parental control for the first time. Drinking isnt evil, but its one of those stupid things that can get you killed, and the age limit gives people a little time to adjust to being on their own.
Besides, its not like Americans are missing much. Most of what we drink in this country is a weak 'beer-like' product; Bud, Coors, Miller. So what if you have to wait four more years to drink Bud. There are outstanding American beers - Magic Hat's Phat Angel, for instance, has a very similar flavor to that of Bass - but most Americans seem to favor beer-flavored water to real beer.