I think I was ten, could have been nine though. A bit young anyhow, and no, no family involved. I had a bit of a shock years later, when some kid I was babysitting ask me if he could suck my tinkywinky (he used a different word, starts with a 'D'). He was only 6 years old at the time and told me that he did the same thing to his 8 year old brother and his brother's friends. I was rude, I guess, by turning him down (something about going to jail and such) and later on that evening had to tell someone about this shocking experience (the lad even had a boner, not more than an inch or two, but still, it showed in his pajamas).
This someone turned out to be my foster mother's sister who was staying at our place with her husband during X-mas. I was 19, she was 30-something and before I could stop her she was warming her hands in my lap (yes it was cold outside, being December and all, in New England), but still, her husband was sleeping only yards away with his big gun within reach (He was a policeman at the time, maybe still is). I turned her down too that evening. I felt very wanted that day, but wasn't happy about it...
Anyway, I thought I was young when I made the big step, but apparently some are even younger.