Cultural thing.
Honestly, I don't know what the massive problem with youths drinking underage is. We all did it; and most of us turned out ok.
The main problem-children seem to be ones who don't drink at all responsibly, or who go out of control. I don't see how that'd be stopped by them turning 18. It just means they can do it inside legal establishments.
This also seems to be a class thing - I've always been brought up to be a social drinker so don't get physically aggressive or abusive when drunk. I'm not sure how you could change these kids and encourage them to do that
Incidentally, raising the age to 18 will just make us like America, where teenagers still get trolleyed, they just do it at house parties instead of nightclubs.