This happened when I had just turned 13
I had just had my 13th birthday and had not learned a thing about the opposite sex, even though I had an older sister by four years. Our house was pretty shut down and I was very late hitting puberty. I had boners all the time and didn't make any connection to masterbation.
This happened at scout camp with guys 13, 14 and one at 15. One afternoon about five got the idea to go off and 'do it'. I was invited along and didn't get the idea, thinking they were going for a hike.
About a mile off into the woods they found a secluded, very private spot and it was clear they had been there before. They dropped their shorts and leaned back on rocks or onto a tree. They quickly they got hard and began jerking off. They had all been circumsized and looked a lot bigger than my pre-puberty penis. I didn't understand what they were doing and they told me to drop my shorts and undies, too, and try it. I didn't have any hair like they did and it didn't seem fair, but WOW, did it feel good. It didn't take long for one of them to say he was coming and then, like popcorn, they began to shoot off long ropes of white sperm. I was amazed to see it and to watch their faces contort with pleasure from doing it. Of course the idea was to see who was fastest to cum. I don't recall who won.
For me, it was a non-event because I would be nearly a year more to fully hit puberty. Also, my cock swelled up and got a little sore from the 'beating'. They did it a couple of more times before camp ended and I never saw another event like it. As I matured I learned the better approach to getting there was to take time and enjoy the trip.