Also aren't you in 'merica where you can't drink until your 21 not 18 like most of the civilized world? Shouldn't be like buying guns and shooting them on your 18th in 'murica instead?
I think the real question is "why does your government trust you to vote for their next leader, bearing in mind that you're the most powerful nation in the world, but doesn't trust you to drink?"
Well the reason why the legal age of drinking was raised from 18 to 21 was because of what was it called.. M.A.D (Mothers Against Drinking) I think the group was called? Because people who were 18 were driving and getting killed because they were also drinking (Even though that happens at other ages too.) and basically got congress to suspend funds for road construction from the states until they changed their state drinking laws to 21.
So basically Parents get mad for people who are 18 are drinking because it is dangerous, but not if they are risking their lives and out shooting people.