Plus drinking a lot isn't good for one's health. It makes people fat and ruins their liver. Plus, it is a waste of money that could go elsewhere. Those are great reasons for you to abstain, but not a single reason for you to choose for others to abstain.
It's not your health, your liver or your money you're talking about. It's someone else's.
I think that when the government writes drinking laws for underage kids, they look for what's best for the young population as a whole. Of course I was stating my view, but I am sure they are similiar to the philosophies behind drinking laws anyway.