According to this article in the New York Times:

• Just 11 percent of Twitter users are aged 12 to 17, according to comScore.
• Almost everyone under 35 uses social networks, but the growth of these networks over the last year has come from older adults, according to a report from Forrester Research issued Tuesday. Use of social networking by people aged 35 to 54 grew 60 percent in the last year.

I particularly agree with this statement from the article, “For Twitter’s future, young people’s ambivalence could be a good thing. Teenagers may be more comfortable using new technologies, but they are also notoriously fickle. Although they drove the growth of Friendster and MySpace, they then moved on from those sites to Facebook.”

The focus on adults may finally give B2B marketers a social network to call their own.  Some may say that this focus already exists with LinkedIn.  I agree that LinkedIn is a truly professionally oriented site, but most see it as a tool for connecting and job hunting, not a place to expound the benefits of business systems.  Twitter gives B2B marketers a tool and an audience that is appropriate for them.