This happened when I was thirteen and afterwards many times as well. My parents are divorced and sometimes when the two of them were out of town at the same time I would stay at my uncle's condo. My uncle was single and in his thirties and he had a nice condo.
I was attracted to him in a funny way as my girlfriends would mention that he was hot. Well once I had to stay at his place a whole week and I would sleep on the pull-out bed in the living room and he would be in his bedroom. The bathroom had two doors, one that lead into the hallway, and one that lead into the bedroom. For some reason the door to the bedroom wouldn't shut all the way.
One night we went to sleep after watching t.v., after a while I went to the bathroom as I forgot to brush my teeth before going to bed. When I went into the bathroom I could see the light from the room shining in and realised you could see into the bedroom. I was very small at the time but had started to develop. I walked quietly toward the door and peeked through and saw my uncle lying on his bed jerking himself, he must have thought I was sleeping.
It was the first penis I ever saw and I was fascinated. He was in good shape too. As I watched him I put my hand into my pyjama's and started to play with myself. His bed was only a few feet from the door and I had a perfect view. I had masturbated before but never watching a guy doing it. He seemed to really enjoy what he was doing and that made me enjoy it more too. I watched him for over thirty minutes, playing with himself until I thought he was going to get up and come into the bathroom, so I ran to the living room.
While I laid in bed I continued masturbating thinking of him. For the next couple of days I did the same thing and each night he would start jerking off before bed. One time I watched him as he stood next to his bed and jerked himself until he came over a towel. This made me really hot and I started to cum as well. All the time my heart was pounding as I knew he would get mad if he caught me watching him. Sometimes in the morning I would walk around in only a loose shirt and panties seeing if it would make him hard. After, he went to work I would lie on his bed and masturbate and think about him jerking as if I was there next to him. That was the best orgasm ever.