Well, I hit puberty really early so I probably would have started around 9 if I hadn't completely repressed myself. Sexuality for me meant admitting I was a girl, and I wasn't going to do that. So I only started around 12. Totally ->-bleeped-<-ed up creepy fantasies. Mostly with me as a girl at first, took me a while for it to occur to me that I could do otherwise, then I'd put a bottle between my legs and treat it like a penis.
Actually, now that I think about it, I did masturbatory type things when I was around 8, I just didn't realise it. Usually without thinking about it, and when I was 9 or 10 occasionally with control/sadism fantasies minus the sexual element.