I believe the rule is due to the fact that they want to guide children and teenagers to a moral path. Drinking itself may not be an issue, but addiction and lack of inhibition could cause recklessness. Teenagers, in general, are very impulsive.

[That said, it wasn't as if adults were much better but...]

It's easier to have control once you're older, I believe, especially when society is as large and intricate as ours.