Twitter’s footprint has expanded impressively in the first half of 2009, reaching 10.7 percent of all active Internet users in June. Perhaps even more impressively, this growth has come despite a lack of widespread adoption by children, teens, and young adults. In June 2009, only 16 percent of Twitter.com website users were under the age of 25. Bear in mind persons under 25 make up nearly one quarter of the active US Internet universe, which means that Twitter.com effectively under-indexes on the youth market by 36 percent.
Swivel has a graph showing Twitter use by age range:
Links to the article with the data: