Some of the stories here remind me that guys just starting out in puberty like to masturbate even though they can't cum yet, and when they hear from a guy a couple of years older what that to cum is part of growing up, they want to know more and are glad for a show sometimes.
When I was about 15, I was hanging out with a 13-year-old friend who lived down the street, and we, as is common with boys, started talking about sex, or what we thought sex was anyway. We both admitted to each other that we like the feel of rubbing our own cocks. Talking about it made us both hard, so I said let's go into the tool shed and compare our equipment.
Once in the shed, we unzipped and each let out his one-eyed trouser mouse. Wow! Although only two years older than my friend, I was surprised to see how much bigger I had grown since I was his age. My cock looked like a man's cock compared to his, and I remember having this awareness of my adult cock for the first time right then and there. Whenever I had seen other guys naked in the showers at school, they were always about my age and just as big. I had not really noticed how big my balls and how thick my cock had become until my younger friend and I stood next to each other with our dicks in hand.
I said why don't we jerk off to take care of our hard-ons, and he was into that. I asked him if he wanted to see what it was like to cum, and maybe he would cum too. He didn't think he'd cum because he never had, but wanted me to show him how I did it and what it looked like.
We both started jerking off, standing next to each other with just our flies open and our underwear pushed down only enough to let our cocks and balls loose. He rubbed his skin up and down over his head and shaft, and I rubbed my skin between my thumb and forefinger while my cock swelled and the head got redder then purplish. Soon I was ready to shoot, and reached the point of no return. My cum shot out and, although I tried to catch it on my dick and hands, some of it spilled onto the concrete floor of the tool shed. I think my friend was really surprised at what it looked like when I shot my load, and he watched my cock closely the whole time.
To this day, decades later, I still see the stain on the floor where I jizzed that warm summer day with my friend watching me.