We were both 13 when this happened
When I was thirteen I went on a trip over the weekend with my class. There were four big rooms that had beds, so two of the rooms were for us guys. At night we talked a lot about everything: school, what we were going to do the next day, and a bit about masturbation. No one would admit doing it, but we talked a bit.
The next day I was going to take a shower, but since there was only one functional shower, we were advised to shower two at a time. So I and one of my friends, let's call him Adam, decided to shower together. We quickly stripped down and jumped in the shower. There were two shower heads next to each other, so we could see each other clearly.
We started to talk about everything. After talking a while, I suggested that we could compare erections, and Adam thought that would be a good idea. We started to rub our dicks and we got hard, and realised that I was a bit bigger than him, but not much. Then I asked if he had ever jerked off before, and he said he hadn't, but I think that was a lie. I lied too, and said that I hadn't, and I suggested that we do it right there. He agreed quickly and we started to rub ourselves. After a while both of us ejaculated, and we both agreed that it was nice. We never did it again, because he moved away over the summer.