I used the college kids example not as a "twitter is meant for younger people" example. It's the fact that college campuses are usually good predictors of technologies and apps that are gaining mainstream popularity. Ironically Facebook is growing heavily in the older segment, which is making the "Facebook stealing twitters thunder" even more important.

Another overlooked demographic of the twitter userbase is the urban segment. A ton of rappers use the service, hence bringing more fans. Go look at most of the hashtag trends on search.twitter ie- #whyursingle. Almost all of the responses are from an urban/black audience. I think there's some X Factor there, as it could turn twitter into something more mainstream. I don't know.