The first time I saw Sean next door 'do it', I had no clue what he was doing. I couldn't see his bed from my bedroom window, but that morning I walked by my window and glanced over, and at an angle looking out my window, I could see him on his bed reflected in the mirror over his dresser. Sean, who is five years older than me, was naked on his back, and would alternate between fisting up and down, and holding his fist still, and thrusting up into it. I had never seen a fully grown penis, and I was mesmerized by the size, and the bright red hair around it. He finished up rather quickly, and I went to my sister Ellen's room, who was 18, and promptly asked her what Sean was doing. She proceeded to tell me in great detail how boys pleasure themselves. I learned that he hadn't 'peed' on himself at the end, and what semen was. I couldn't understand why if it felt so good, why his face looked like he was in pain. Ellen made me promise if I saw him do it again, to come get her.
A couple weeks later I saw him doing it again, so I ran and grabbed Ellen. We watched him squirm, arch and shoot, and Ellen was rubbing her crotch as she watched him. I asked her what she was doing, and she said she had an itch. Ellen went back to her room, and not long after, I went down to talk to her, and heard her bed squeaking, and the door was locked. I knocked and she said to go away she was studying, and it wasn't until years later that I put it together that she was in there getting off on the sight of watching Sean. Later on I had discovered the joys of masturbation myself, and Sean was a senior in high school, still jerking off at every chance he could get. The first time I fingered myself while watching him left me with an incredible orgasm.
The hottest time of all however was one Saturday afternoon, when I looked over and saw him making out on his bed with his girlfriend. They ended up naked, doing oral, and then having sex. I was dripping watching them, and was furiously fingering myself. I climaxed so hard, I thought I was going to pass out. I came much faster than they did, and actually ended up bringing myself to a second orgasm as Sean came in her.
All this happened over 40 years ago, but it still gets me horny thinking about it.