It should be 18, by all means. Why?

Because most people start drinking around that age, in college. And if they can't go to bars or places where it can be monitored by adults/RA's/whatever, they're going to do it underground, and that results in a lot of dangerous things going on, like date rape, binging, and drunk driving. It's not safe.

There is no reason that it should be any higher. I can't think of a reason for it to be lower - I think teenagers should be allowed to drink in their house under the company of their parents if they allow them, and their parents only (not other parents), but that's pretty much what goes on as is.