I started at 12. All my friends admitted doing it and some bragged. The ones that degraded it saying it was not good, were the ones that were not able to cum yet and embarrased about it probably. But don't push the subject with them, just enjoy doing it yourself and be glad you're already old enough to enjoy it!
I agree with the other Andrew. People tend to start early but don't admit it until they are more mature and comfortable with themselves. It's kind of like the classic gym class example--13, 14 year olds tend to be more self conscious about changing or being shirtless b/c everyone is at a different level of development. But I've noticed this year, in a class of 16 and 17 year olds, that since everyone has armpit hair, no one minds being skins. And since people are fully developed, no one worries about showering. With masturbation, once everyone is doing it, no one makes a big deal of it.
Getting back to the point, I was 11 or 12 my first time. My guess would be that most people, especially those under 15, start out just by playing with themselves and cum by accident their first time.