This happened to me when I was around fourteen.
When I was younger, around fourteen, I had a friend (we'll call him Jake) who was about three years younger than I was. We went on a camping trip together with our dad's for a week.
While they were playing golf one day, Jake and I were bored so we went off and sat by one of the small lakes. We were just chatting about things and we ended up getting onto the subject of sex and masturbation (I can't remember how.) Anyway, I asked him if he knew anything about it and he didn't. I'd been masturbating since I was about eleven, around his age then. So I went through all the details about sex to him, and as you can imagine it was making me fairly horny describing the subject.
There was an outdoor toilet nearby so I suggested that we both go in and give it a try. Since I'd described how he was supposed to do it, he went inside first, but came out a minute later saying it didn't really do anything for him. Since it would have looked fishy if I went in there with him I just dropped it.
Later that night we were both in the tent by ourselves and I brought the subject back up. I used to masturbate by lying face down on my mattress and rubbing myself into it until I came. I suggested he try this too. I don't think he really managed anything, but I went at it next to him and managed to cum in my PJ's. I didn't tell him though.
The next morning we were in the camping ground showers, there were about six separate showers (all closed off to each other) and an area on the opposite side with benches. Anyway, before we both went in I suggested he try it again. So we were both in the showers opposite each other and both tugging away on our dicks. Neither of us managed to cum (I was used to the mattress then and completely unaware of lube at the time) so we forgot about it for the rest of the camping trip. As you can imagine as soon as I got home I locked myself in my room and humped my mattress until it was soaked.
About a year later Jake came over to my house. We hadn't seen each other for a while. The first thing on my mind was wondering whether or not he had successfully masturbated yet. So we were in the backyard sitting next to the pool and I asked him. He told me he hadn't really tried since the camping trip. So I suggested (now that we were at home and alone) that I show him how I did it.
We went into my room and I closed the door just in case. At that time I had an inflatable bat that I used to lay down on the edge of my bed on, and hump until I came. Because it was only about 80% inflated, I could bend it and press it into myself as I went.
So I got the inflatable bat out, dropped my pants and just started humping it, with him standing at the door watching. I don't know if he got turned on by watching me or not. I was naive and incredibly focused on myself at the time. After about a minute I came, and he had a clear view of my cum spurting onto the bat. I had the perfect chance to offer him a go at that stage or to help him out, but because I had just cum and I was only young, I started feeling guilty and suggested we go and play a video game instead.
I moved away about six months after. I still wonder what could have happened if I had offered to help him learn too. I don't know whether he ever managed to successfully masturbate after seeing me do it. I like to think that I helped introduce him to the world of masturbation though.