Social network users overall skew young, but when looking at specific sites, differences emerge between age groups.

In June 2012, Pingdom analyzed DoubleClick data and found that 26% of US social network users were between the ages of 25 and 34, with another 25% between the ages of 35 and 44. Users ages 18 to 24 made up 16% of all US social network users. This study looked at users of a variety of sites, including Facebook, Twitter, LinkedIn, Pinterest, Tumblr, Reddit, StumbleUpon, deviantART and Goodreads.

via Age, Interests Dictate Social Networks Used – eMarketer.