Most children biologicly dont masturbate till they get 12, some of them do it earlier, for example u have kids whos puberty starts premature at 9 or 10, and second of all they could be prematurly stimulated, by friends ( often a few years older ) or an parent or an older brother/sister wich for example leave porn books laying around and the child finds them,,
On the web i know one woman, who claims thats she fingererd herself till an orgasme at the age of 9....