Why do you think the drinking age makes 21 year olds behave like 16 or 18 year olds?
I certainly don't want to call my fellow Americans particularly mature, but it's pretty rare for people to wait until 21 to drink here, so I think the effect is dispersed.
More recently, they've started to crack down on underage drinking, and now the police blotter fills a column in the campus newspaper with a series of "Driver arrested on suspicion of DWI." Hopefully this is a result of them cracking down on drinking and driving too, but I fear that to some extent it's because they are forcing the parties off campus.