Pew Research Center: Twitterpated: Mobile Americans Increasingly Take to Tweeting.
This article from Pew has a lot of Twitter demographics:
A look at the demographic profile of Twitter users as a whole reveals some additional details about who uses Twitter and how they communicate and consume information.
As noted above, Twitter users are overwhelmingly young. However, unlike the majority of other applications with a similarly large percentage of youth, Twitter use is not dominated by the youngest of young adults. Indeed, the median age of a Twitter user is 31. In comparison, the median age of a MySpace user is 27, Facebook user is 26 and LinkedIn user is 40.5
Twitter users are slightly more racially and ethnically diverse than is the full U.S. population, most likely because they are younger — and younger Americans are a more ethnically and racially diverse group than is the full population.
Twitter users are also slightly more likely to live in urban areas, with 35% of Twitter users living in urban areas (compared with 29% of all internet users) and just 9% of Twitterers and status updaters living in rural areas, compared with 17% of internet users.