65% of American Facebook users are aged 35 or older

"55% of Twitter users are 35 or older.
63% of Pinterest users are 35 or older.
65% of Facebook users are 35 or older.
79% of LinkedIn users are 35 or older."

Source:  Data from Pingdom, reported on their site, 21st August 2012
Note - More on the demographics of Facebook, the demographics of Twitter and other sites on the Pingdom site