Here's an article I just read on Mediapost regarding teens and Twitter. It pretty much confirms what everyone already knows, namely that teens don't care one bit about Twitter and prefer Facebook and photo sharing sites instead.
But then the article goes on to discuss how to make Twitter more relevant for the teen audience, basically saying teens could use Twitter to share photos and videos instead of (or in addition to) using Facebook and YouTube to do the same thing.
Personally I don't see the day when teenagers will find any use for Twitter (unless they own a business or have something to promote). Twitter just isn't built for the teen market.