Most teens have much smaller network of friends than adults do. They are also less interested in  knowing what's going on beyond their immediate surroundings. I think these and other differences like less self-confidence and higher sensitivity to privacy point to weaker demand among teens for microblogging services like Twitter in current form.
But same arguments could be made against MySpace yet it is supposedly popular among teens. Is it? Other than stats, it's difficult to really know. My son is aware of MySpace but he frequents Lego enthusiast forums and YouTube much more than MySpace pages. My son didn't know what Twitter was so I explained but he didn't find what Twitter does interesting.
While I don't have anything conclusive to say and am too lazy to chase down research grants, I think the effect of age (and sex) on social-whatever is an interesting as well as important research subject.