Coming from a family with severe alcohol problems, I'd suggest that the 21 and up rule is for everyone's good. My father died young because his parents were fine with their children drinking from a young age -- he couldn't handle it, and I could fill pages of the devestating results for me, my siblings, my mother . . .
Don't forget that bodies aren't finished growing on the inside by 19 -- especially young men's bodies. A little bit of alcohol "on a special occasion" does send the message that it's okay, and suddenly -- even with the best of intentions -- a little can become a lot. You won't find a single person who set out to become like my father, but you'll find plenty of people who thought they could handle "just a little." It's playing with fire.
What's that old saying? If you wreck your body, where will you live?
I don't mean to offend anyone, but this is a subject that I've lived through -- painfully lived through. Consider it carefully.