18 year olds are asked by their country to go to war, to kill, and to put their lives on the line. But when those same 18 year olds return to the United States, they are told they cannot have a beer because they aren't responsible enough to drink at that age.
At 18 you're an adult. You can vote. You can get your own place. And yes, you can join the military and bomb the hell out of other countries, and get bombed in return. So if you can die for your country, if you can kill for it, why can't you drink in it? Are we saying that someone capable of committing state-sanctioned murder isn't responsible enough to lift a pint with his buddies at the bar? That's just ludicrous.
An argument can be made that if you're old enough to sign up for the military and die for your country, then you should be able to buy a drink. However, it has been shown that drinking before the brain is fully formed can harm brain development. Drinking age should be older to prevent this as much as possible.