Lots of fun stories about early experiences here, so I thought I'd share mine.
As a kid I shared a bedroom with my older brother. I think I was 7 and he was 12 when I first noticed him masturbating in bed at night. He'd wait until he thought I was asleep, then pull his covers down, pull his boxers off, and go to it. I would lay there pretending to be asleep, with my eyes only slightly open. He seemed to alternate between humping his pillow and using his hand. I didn't know what he was doing, but I could tell from the look on his face that it felt good. It took him about 10 to 20 minutes to complete the act, and I could sense the tension building up until he tensed and held still. I could see his penis twitching, and his mouth would be open. He'd lay there relaxing for a few minutes, then pull the covers up without putting his boxers back on, and go to sleep.
I don't think he was shooting semen when I first started watching, but I do remember at some point seeing stuff come out. At first it was just a little watery dribble out of the end of his penis, but later he would shoot jets of it onto his stomach, and sometimes all the way up his chest. It was a magical sight.
I enjoyed the show every night, and got a little boner while watching. I didn't dare do anything with it for fear of him knowing I was awake and watching, but I did play with it at other times and knew it felt good. But it wasn't until several years later that I had my first orgasm and fully understood how good my brother felt when he did that.
Some nights I fell asleep before he was done, and wake up in the morning realizing I'd pooped out during the nightly show.
This went on until he moved away from home, and I was at that point doing it. Sometimes I wished I had a little brother to watch me like I had done, and that was one of my recurring masturbation fantasies.
I'll send a second post to tell the story of my first orgasm, and what happened then.