This happened when I was twelve.
I was at my grans house with my male cousin who was a year older.
My gran was going into town, she said that she wasn't going to be long and that we could go with her. Neither of us wanted to though so we stayed.
My cousin has always been quite sporty and aggressive and he asked me to wrestle with him. I didn't want to but I said yes because I knew that he would only get angry. He said he needed a minute to get ready and asked me to get out of the room. I did and then he said that I could come back in.
I was very shocked by what I saw. He was totally naked. My eyes were instantly drawn to his penis. Even flaccid it was about three inches. It quckly became erect like mine. When erect he was about six inches, an inch bigger than mine. He quickly grabbed his shaft and began stroking. I quickly copied him and within five minutes we were both about to come. We began to moan and my cousin shot a big load. Then I came with one of my best orgasms.
We both collapsed onto the couch. I said to my cousin, 'That was great wrestling'. We both laughed and agreed not to tell anyone. We did many things after that and I will write about them sometime.