But myYearBook CEO Geoff Cook has a strong analysis on Techcrunch that dispels some of the myths. Largely what he finds is that teens are disproportionately on Twitter. The sense that teens aren't Tweeting is no different than the sense adults aren't Tweeting --- a large majority of each isn't.
What Cook says, though, is that teens aren't Tweeting for the same reason adults aren't --- that they can do what Twitter does elsewhere.