Before John Doe may legally order a beer at his local restaurant:
-He may purchase tobacco
-He may drive a car without restriction
-He may own a semi automatic rifle in many states
-He may enlist in the armed forces and die for this country
-He can (and very well may) purchase a forged federal document in order to obtain alcohol illegally
-He is likely to develop addictive or corrosive drinking habits upon finally being exposed to alcohol legally.
Find a law that works. If we, as a nation, decide that 21 is the right age, so be it. As of now, laws are absolute in some areas, yet completely ignored in others. Children with high hopes of the American Dream can have their lives destroyed by a simple underage drinking citation.