i started at about 10 or under.
haha i have the best story as to why
in gym in like beginning 4th grade, we climbed ropes (the classic climbing ropes story) and it made my dick feel good and warm, and then i always had to pee. lol it must be funny to be gym teacher and know that kids get boners off of that, and then get asked if they can go to the bathroom. but so we were learning about the friction and stuff and i applied that to the gym thing.
i basically learned on my own, at first i did 2 handed and i thought that was normal. to answer that guys question to different methods, lol try going opposite hand, or over hand (turn your hand upside down) lol im getting pretty ambidextrious nowadays.
i started rubbing myself when i was like 8 though, just unaware of what it was. just felt good