http://www.quantcast.com/pornhub.com
Age stats, 19% under 18.
http://www.alexa.com/siteinfo/pornhub.com
Click on audience
Now if 1 every 5 adult site surfers is under 18 (no matter they click "I am over 18" button), you could expect at least 1 every 5 adult affiliates it starts when under 18.
Those here who started when older (includes me), are simply born before 1980 and so there was no internet yet when they was under 18.
In fact, I did operated a BBS (bulletin board system) in 1990 when I was 15, it was not adult theme anyway - just to say the 14-16 year olds have way more free time to setup online stuff then anyone else.