According to the Pew Internet & American Life Project the percentage of online teens who blog has dropped 14 percentage points in three years, from 28% in 2006 to 14% in 2009. Blogging amongst adults has remained steady, at roughly 10% over the same time period. Behind that number, however, Pew finds that blogging by young adults (18-29) has suffered a steep decline, offset by a rise in blogging by those of us on the wrong side of 30.
You can find the full report — “Social Media and Young Adults” — here.