It has nothing to do with our level of maturity, or how responsible we are. It's just the way our society views alcohol and when it can be consumed by people.
For some reason we believe you can't drink until 21, even though we are adults at 18.
It should really be lowered, than kids wouldn't be going crazy like they do nowadays (I guess I'm apart of that group, I used to booze my brains out under 18, and I still drink pretty heavy when I do
).
My senior year in High School the German exchange student (Total hottie haha) was surprised at how American teens partied. I guess in Germany you can drink beer at 16, hard alcohol at 18. She told us they'd go to the clubs to have fun, not to get smashed, and drinking was more for socializing than to get intoxicated.