Royal Pingdom has used Google Adplanner U.S. data for 19 different social network sites, and analyzed the data for age demographics. The analysis showed that the average social network user is 37 years old. LinkedIn has the highest average user age of 44, while Bebo has the youngest, 28. The majority of 64% of Twitter users and 61% of Facebook users are aged 35 or older.
Lots more statistics and charts at Royal Pingdom.