IMHO Twitter is a tool for following interesting people who aren't close enough to you to be your friend in something like Facebook. But what makes someone interesting on Twitter? Being a celebrity is not it: Most celebrities aren't famous for generating Twitter-type content. They're famous for acting or sprinting or golf or good looks, none of which translates well to Twitter.
Being an important company is not it, either, unless you enjoy reading press releases or are a serious power user.
The people I follow are hackers, writers, artists, and good linkers. People who are otherwise hard to follow via mass media, but are not close enough to be personal friends. It doesn't surprise me that the audience for that is large but not gargantuan, just as it doesn't surprise me to know that the market for novels is an order of magnitude smaller than the market for sitcoms.
It also doesn't surprise me that the Twitter audience skews older. Isn't catering to kids what the rest of the media is for?