I just read a story on MSN about lowering the drinking age: http://www.msnbc.msn.com/id/20249460/
It seems obvious to me that lowering the drinking age will get 18-21 year-olds drinking beer instead of liquor. The reason they drink hard liquor is because it is easier to get and more effective by volume at getting them drunk. If they could buy beer legally, then they'd be able to get some beer and drink it in a normal adult way.
OK so maybe they wouldn't drink it in an adult way, but they'd be more likely to drink beer and less so to drink spirits. As the article states, look at prohibition as an example. Did the bootleggers make beer? Hell no, they made booze. Why bother transporting beer when you could move something 5-10 times as potent in the same weight and volume.
Let's face it, making something illegal doesn't stop it. People still do drugs - all the time. Kids still smoke - unless they really understand why they shouldn't. And people will drink alcohol when they feel like it, no matter how old they are.
Legalize it, don't criticize it.