By the time I was 12 I actually already knew everything there was to 'know' about sex without experiencing it (though I got my first blowjob at the age of 5, long story short; it was just playing games with the girl next door). Except one thing; jerking off. Though I knew about cumming and whatnot. My parents never sent me out the room or covered my eyes or whatever when explicit topics were on TV or during sex scenes.
Anyway, at 11 I got internet on my computer and at 12 I was downloading internet porn and watching porn on late night TV. I noticed my cock getting thicker, harder and longer when it was erect more than before, and really wet, so I played a little with it but I never full-on jacked it off. A few days later I'd download pretty much any porn and one of those was that softcore Carmen Electra clip on KaZaA. I started rubbing my cock more and at one point I squirted all over the place. I mean, it felt amazing, but I was little like 'holy SHIT'. I was freaked and kinda felt guilty. I couldn't believe that I could do it already. Cleaned the mess up and decided not to do it again. My dad came home like 10 minutes later and I felt like he 'knew', though obviously he didn't.
Anyway, I waited for a day and I started jacking off multiple times per day since then. So much for 'not doing it anymore'.