I'm at a seminar in Virginia, and a fellow here will be turning 21 tomorrow. He will not be allowed to drink until midnight tonight, since, you know, he's such a different guy now than he will be then.
Someone pointed out to me, though, that that's an issue with all age limits/age of consent rules. And you know what? They're right.
Perhaps a different system would be that once someone wants freedom from rules there could be some sort of test of understanding and rationality that they could pass to distinguish themself as a rational individual. It's difficult to implement but certainly less absurd than choosing an arbitrary age.
That isn't to say that I would use that as a rule to determine when someone should be allowed to drink - I don't think age or rationality should have anything to do with that - this would apply more to, say, contracts.