When I was growing up I had a very close friend who was a boy. We did everything together.
When we were both twelve or thirteen he asked me if he could ask me a personal question. I said sure. He asked me if I had ever seen a boy naked. I said no (truthfully). So, he asked if he showed me his if I would show him mine. I thought sure, OK, we hang together all the time, so seeing Derek nude would be OK.
It was a bit awkward, but he took off his shirt and pants and I took off mine. You could tell he had a stiffy by the way his underpants were tented out in front. I slipped off my bra and panties and he took off his underwear. I asked if I could touch his penis and he said sure, if he could get a real look at my pussy. So, I laid down and pulled my lips apart and showed him all my bits. He loved it. I touched his penis and took a real close look at it.
I then asked him how often (not when) he masturbated. He said at least twice a day. I told him I did it maybe once or twice a week. We then offered to do it in front of each other. He grabbed his penis by his thumb and forefinger and just rubbed the loose skin up and down. I circled my clit with my forefinger. About 60 seconds later he came and spurted cum onto his stomach and the little pubic hair he had. I came about four or five minutes later.
He thanked me for being a good sport and I told him I liked it too. We did it together many times over the next couple of years, (I eventually let him watch me shave off all my pussy hair).