If you have the time, be sure to check out David Hanson's wonderful site, Alcohol: Problems and Solutions, especially the wonderful article The Legal Drinking Age:Science vs. Ideology. Here is a choice quote from Dr. Hanson:
"People become responsible by being properly taught, given responsibility, and then held accountable for their actions. We don't tell young people to "just say no" to driving, fail to teach them to drive, and then on their 18th birthday give them drivers licenses and turn them loose on the road. But this is the logic we follow for beverage alcohol because neo-prohibitionism underlies our alcohol policy."
Here here! The doctor is in, and he recommends a prescription of logic and responsibility.
I'll drink to that.