When I was younger, my parents started hiring a certain older teenage babysitter for my older brother and I when they went out for the evening, which they did infrequently. She must have been about seventeen at the time, and was fully developed and pretty forward.
One night as she was checking on us in bed, my brother developed an erection in his pyjamas. The sitter seeing this pulled down his pyjamas and stroked his dick until he had a dry orgasm. I was watching all this from my bed and of course when the sitter had finished my brother off, I insisted she did the same thing for me.
The sitter told us not to tell anyone, especially our parents, about the fun thing she did with us. We may have been younger, about thirteen, but we weren't stupid.
After that the sitter would always play with us before seeing us to bed. She would let us fondle her breasts and play with her nipples while she played with us.
My brother and I had always played with ourselves, though we had not known how to work up to an orgasm before. Once we learned from the sitter, we started playing with ourselves, and with each other, in earnest on nights when my parents stayed home. We slept with the door to our room closed and once the lights were out we would crawl into bed together and have fun.
One night the sitter took my brother's cock in her hand, spat on it to lube it and brought him to orgasm that way. Of course, after she had done it to my brother she had to do it to me, it felt great.
Eventually the sitter brought her boyfriend over. She masturbated him in front of us so we could see how he shot his wad. We were amazed at the size of his cock and, of course, we'd never seen the sticky white stuff before. The sitter's boyfriend explained what it was for and how we would eventually mature in a few years so that we would have big cocks and spurt semen, too. Then he put his hand up his girlfriends (the sitter) skirt, and rubbed her between her legs while we watched. She closed her eyes and moaned. We wanted to touch her as well, and after some persuasion she allowed my brother and I to finger her. I couldn't get over how wet she was and she started breathing heavy and moaning. Her boyfriend masturbated himself whilst watching us bring her off, and shot another wad of cum.
After that, the sitter started bringing her boyfriend over regularly. Over time, she and her boyfriend gave my brother and I a complete sex education, demonstrating just about every conceivable act and position.
On nights when her boyfriend didn't join her, our sitter would let us try some of these neat things on her. And, within the limits imposed by our anatomies, my brother and I tried many of these things with each other whenever we had a chance.
By the time I was 17 and my brother 18, we had done just about everything there was to do with her and ourselves. We introduced some other kids, both girls and boys, to these activities, too. But we knew enough to be very, very careful and we never got caught.