This was a true story that happened in Southern California back in 1975
I played percussion for my school orchestra and band and took private lessons as well. My teacher was putting on a concert for some some of the local schools and asked me to join the percussion ensemble group he formed. There were six of us plus my teacher.
The rehearsal was about an hour from my house and my parents dropped me off. I was getting a ride back with one of the older players that lived closer to me. The player, Chad, who was in high school lived near the rehearsal hall, and during a break we all headed over to his house. It turned out he had a swimming pool. Some of the guys had brought swim suits and Chad lent me an extra one.
When it came time to get into my suit, one of the older college guys, Brian, joined me in the pool changing room. I was small for my age. Maybe 5'2", around 100+ pounds and had a slim build. I was taking my clothes off, and as I slipped my underwear off and reached for the swim trunks, I saw Brian's reflection in the changing room mirror. He was standing, still fully clothed, behind me. He said something like, "nice package". Maybe he was expecting me to be more early adolescent in appearance. In any event, I was a little unsure what to think of his comment and just stood there.
He told me what a nice body I had and said I had been running around all morning and need to just relax some. Brian turned on the faucet in front of us and ran some warm water. He reach around and lathered his hands with soap. I just continued standing there, like a deer in the headlights. Then, standing behind me he reached around with both hands and began to gently fondle my balls and cock with his soapy hands.
My penis quickly hardened and pointed straight up. Again he made some comment that made me think he was surprised at the size of my erection (which was probably around 5.5" when I was 15. I stood there and looking at the mirror I could see myself at full attention and watched Brian fondle my now soapy smooth sac in one hand and run the other over the length of my hard shaft. It didn't take long before I could feel myself on the verge of cumming. Brian continued to jerk me off as the cum shot out of my cock and into the sink and counter in front of me. He rinsed his hands off and towelled me off.
I never said a word, still too shocked by the whole experience. It did chill me out some though.