Freud talks a lot about infants touching themselves and playing with themselves and about how the way that parents handle that can have lasting consequences. Apparantly some parents say things like 'if you play with it it will fall off!' or 'that is dirty!' and that can result in the kid feeling really ashamed about that. or if the parents react with disgust or by slapping their hands or whatever. sometimes religious beliefs can impact too.

i haven't spent enough time around kids to know...

and i don't have memories before i was 14 but that doesn't mean i didn't do anything until then...

Kaplan and Saddock (in their book on psychiatry) say that you shouldn't ask 'do you masterbate'? you should ask 'how often do you masterbate'? because people tend to say 'no' in response to the first question but are more likely to give an honest answer in response to the second.