if I had to make the argument, I would base it around it around these points and keep it simple while trying not to sound, like Zero said, giddy:
at 18, you can vote
at 18, you can serve in the armed forces
at 18, you can drive (most since 16, but you get the point)
if you're legally responsible enough to do these things, why shouldn't you be allowed to drink? I would put a heavy emphasis on voting, and how the age to do that was lowered. Finally, I would question why it was raised in the first place, and give examples of why those reasons are no longer valid.