When I was fifteen, my mother and I were going on a boat trip, over a weekend. My mother's best friend and her son, who is my friend and a year older than I, were also going.
The first night nothing happened, we just talked a bit about everything. The second night however, we started talking about masturbation. We talked a lot about how often we did it, when we started, and all of that. Then I suggested that we should see who could jerk off the fastest. I said that we should take turns in the bathroom that was in our cabin. All cabins had bathrooms, since it was a really big boat.
So, I went in first and jerked off, and when I was finished he went in there. We didn't do anything else, we just talked a bit more. Though I think that he would have been open to trying mutual sessions, I was too afraid to suggest it (and not horny enough, seeing that we had both just ejaculated). I did however hear him masturbate early in the morning again.