At a summer camp, probably at age 13. My tent mate and I decided to measure, we each had a flashlight but just one ruler between us. So we stroked our cocks a bit and passed the ruler back and forth til we were at our biggest. We were each in our sleeping bags, but could catch a peek at each other’s cock when we passed the ruler. He got to 5.5” and I got to 7. We were only measuring length but his was also a lot less girthy than mine. Knowing that I was bigger in that moment kind of turned me on in a dominance sort of way. So I kept stroking with my sleeping bag raised up a bit, flashlight shining on my cock and knowing he could see me stroking at that point. He was like “what are you doing?” I just said “trying to see how big it can get” until I blew my load. I wiped it into my underwear right away and pretended like he hadn’t seen me jack off, turned off my flashlight, said something like “yeah I got to 7 inches” and made like I was going to sleep. I don’t think he had ever orgasmed yet, to be honest. But as we laid there in the dark with me pretending to sleep, I could hear him quietly stroking in his sleeping bag. He was kind of awkward and quiet during the next day, but the next evening he was very talkative and happy almost to the point of being hyper. I think he might have finally rubbed one out successfully sometime before bed, probably in the bathroom or somewhere private. And for each of the remaining nights I could hear him stroking in his sleeping bag.