I was 16 when this happened
So my neighbours hired me to babysit as they had deemed me responsible enough, they left at 7pm and their 14 year old son (Mitch) and I were watching the rest of a movie until 9pm when he went up to bed.
At about 9:30 he came downstairs again, naked with an erection. I told him to go back to sleep and put some clothes on, but he asked me to help him masturbate, I didn't know how to handle this situation so I just asked him to go upstairs again. He continued to say no and told me that his friends were masturbating on a sleepover but he didn't feel comfortable taking part and that he wanted to see what it felt like.
Since he was refusing to go to bed I thought that if I did it then just let him go to sleep then he'll leave me alone. So I told him to go upstairs and I'd come back over in a bit. I came back over with some lube and proceeded to give him a handjob, a couple of minutes in he started feeling me through my jeans, and I was hard at this point and he told me to stop and take off my clothes. I don't know why I obliged but I did, and we were masturbating one another for about 10 minutes then he started shivering and had got his pleasure but didn't cum.
He told me to lie down and that he would pay me back and pumped me for another 20 minutes before I came. After I came he said he enjoyed this and wanted to do it again when I next babysat. A year on and we're still doing this.