The Changing Demographics of Facebook and Twitter Users
In 2009, the number of Twitter users increased by 900%, from just 2 million users in 2008, to over 20 million users by the end of 2009. The Twitter user demographic that increased most rapidly in the past year is the 18-24 year old set. This shift results in a fairly even split between Twitter users who are over and under the age of 35.
Source: Jamiepappas.com, Comscore 2009 US Digital Year in Review