I was about 8 or 9 years old. My younger cousin, Robbie, and his friend "Scooter" came over to play at our house for the day and were planning on staying the night. We were sitting out on the back porch and they were talking about needing to go inside to do their exercises before bed. My brother and I wondered what they were talking about and so we went down into our basement so they could show us.
At this point, both of us were completely naive to any use for our penises except to go to the bathroom, so we were both surprised and curious when our cousin and friend simultaneously dropped their pants in front of us and begin to rub themselves.
I asked why they were doing that and Robbie told us it was an exercise that would make your penis bigger. While I pondered why in the world you would need it to be bigger, he quickly added that it felt really good too. He said it was called "one man sex". With a minimal amount of prodding, my brother and I pulled down our pants and soon enough there were four boys standing with our pants around our ankles watching each other masturbate. I was impressed because I did get a little bigger and harder too. That was the first time I'd ever had, or at least noticed I had, an erection. And he was right. It felt pretty good too. We exercised together on a pretty regular basis after that day.
After we'd done this a few times over the course of the summer, one time Robbie said he was too tired from playing and still needed to do his exercises so he asked if I would do it for him. I didn't see anything wrong with it so I did. We were so naive back then. It was a lot of fun and soon led to other "exercises" that involved our mouths but that's another story. As it is, I think I need to go do some exercising just about now.