This happened about two weeks ago.
My friend and his wife were out of town and the sitter had to leave before they were back so there was a several hour span their kids would be home alone for after work. I told them my wife or I could come over before they got back from school and hang around until they got back.
They have two sons, 15 and 10 and our families get along great. I left work early and went on over to the house around 2pm, the kids are usually back aroun 2:30-3. I let myself in and made myself comfortable on the couch. I got a call from one the youngest's friend's parents (still with me? lol) and they said he went to their house (he as the main reason for needing someone at the house). Being already there, I figured I'd hang around to make sure the 15 year old was fine. He eventually arrived and we talked for a bit, then he said he was going to his room.
I just hung out downstairs, did some work, relaxed, etc and by now it was around 6pm so I figured I'd go upstairs to see what the boy wanted for dinner. I made my way to his room, and as I got closer to the door, I heard some soft moaning. I didn't think anything of it and continued on. Getting to his room now, I couldn't believe it, but he was laying on his bed, door wide open, completely naked and masturbating. At first I was shocked and quickly got out of sight. My interest got the best of me however, and I peaked back inside. I noticed his eyes were closed and one hand was rubbing his balls while the other slowly stroked his cock (avg. size for a boy his age, maybe a bit shorter). I stood there amazed that he'd be doing this with the door wide open but then I recalled when I was his age and how I'd be in my own world when the urge struck me.
I continued watching and I admit I was beginning to get hard as well. I slowly rubbed myself through my pants and noticed his pace was getting faster and his breathing harder. I knew he was close, so I hid myself more but kept watching. Soon enough his body was shaking and he began to cum thick ropes on his hand, dick, and chest. I was shocked by how much he shot! He opened his eyes at this point and I quickly backed away (still not sure if he saw me or not).
After watching this, I was fully erect and proceeded to the bathroom. Once inside, I stripped my pants off and began to stroke my throbbing penis. It wasn't long until I too began moaning and shooting ropes of cum all over.
I left the bathroom and made a considerable amount of noise upon getting to his room so he'd know I was there. We chatted (not about anything in particular) and that was that.