I think the argument was that it would reduce the 16 (or thereabouts) population drinking and that they're the 'unresponsible' element, while having more 21-year-olds (and over) around would keep the younger elements in check.
The guy was saying that after the drinking age was lowered, it brought younger people into pubs which then reduced interest in older people etc. etc.
Like I say, I don't agree with it.
--------
The problem with your proposal is that drinking is seen among several (though not all by any means, I wish to make clear) young people as an end-in-itself: I know plenty of people for instance who often go out purely for the purpose of getting drunk, and I'm just recently 18. It's almost seen as an 'adult' thing to do by some people.
So while you'd certainly get plenty of people in your converted bar, the people who are causing problems will pursue alcohol regardless.
Thus, the way you change behaviour is by changing the way people see alcohol/drinking so we're back to - ta da! - education.