I was probably no more than 7..

I remember lying on my stomach in bed and noticed how moving my pelvis around felt good (and better and better) until I 'came'. At that age I had no idea what had just happened. I also remember the orgasm felt slightly painful. Over a couple of years or so I switched to the standard jerk off method instead of madrass humping. Nothing came out until I was about 12 I think..