I'm fairly certain that I (and others) played with my penis early on (kindergarten) as I have fragments of memories of some of those experiences. Got into "trouble" several times for doing that though I didn't really understand why at the time. It just felt good.
But in terms of masturbation to produce an orgasm, I was either 11 or 12 (certainly no older than 12). We moved into our house when I was 10 (5th grade) and at first my brother and I shared a bedroom. Sometime in the next year, my dad moved me into the room that was initial designated as "the den." It was either at the end of 6th grade or between 6th and 7th grade.
The first time orgasm I remember was entirely accidental. I had taken a shower after school and came into my bedroom with just a towel. I closed the doors to my room. I had a painted wooden semi circular chair for my desk. After finishing drying myself, I didn't get dressed. Rather, I sat down naked on the chair to start studying. The chair was cool against my skin. After studying naked for a while, something possessed me to take my boy-sized penis and stretch it as long as I could and then fold it under myself between the chair and perineum. Now, I can't remember whether I had done this before, but with my penis under me in this way, it started to get harder and warmer but couldn't get erect.
As it lengthened a bit more under me, I rocked and slid forward and back very gently. The glans was turned down to the chair's surface and as my penis became more turgid, I could feel the underside of the tip in contact with my anus (which I liked). The underside of my penis was rubbing my perineum and it all started to get sort of tingly. I didn't know what the sensation was but I knew I liked it. There was a bit more friction on the wood surface and that was a bit annoying at first. But as time went on, there was a bit of wetness that was helping things to get quite pleasurable. I didn't know that there was pre-cum fluid leaking out and helping with lubrication.
The next thing I knew, I was gushing out this liquid underneath of me. I was pretty sure it wasn't urine and I wasn't peeing. It immediately helped with the friction aspect against the chair surface and it was very warm as it squirted between my ass cheeks and all around my anus. I tingled and then shuddered all over as I leaned back on the chair to enjoy whatever this was. By leaning back, the head of my penis was putting gentle pressure on my anus. After about 30-45 seconds, the intensity began to subside. Shortly after that, my penis became less turgid and wasn't "straining" to become erect. I stood up and looked down at the chair. There was a long ribbon of this sort of cloudy looking whitish fluid on the chair and between my ass cheeks that smelled a bit like chlorine.
"Wow, that felt good" I thought. I wasn't sure what had happened but I sure did like it. Was this the sperm I had been told about? It must be, but at that age I wasn't sure. I wondered if I could do it again. I hadn't wiped the chair or between my legs. So, I folded my penis between my legs and sat down once in the same way that I had before. This time my cum made a nice lubricant as I slid and rocked myself back and forth on the chair. It took longer for the sensations to begin to build and they built at a slower pace this second time so I could notice the sensations as they built. Of course, the other part of the stimulation was the anal stimulation because the head of my penis was rubbing my anus and this time everything was quite slippery. The anal stimulation was quite pleasurable. Again (but not as fast), I felt the sensations building and then the release and more jets of cum between my legs. I felt pretty spent after that. The head of my penis was pretty sensitized after all of this.
I didn't go and clean up right away. I laid down on the damp towel on my bed as I allowed myself to wind down. Its coolness was nice against skin that had gotten pretty hot. I might have dozed off briefly while on the bed. I'm not sure. I eventually got onto studying.
Eventually, I did figure out that it was rubbing the underside base of my penis that was the most stimulative. And I pretty much masturbated everyday and sometimes two or three times. As I got more familiar with the sensations, I also trained myself to build to orgasm slowly and to extend the enjoyment over time. But that first time I remember cumming was quite memorable from the standpoint of what happened and how it happened. It was still a number of years before I surrendered my virginity (I didn't lose it, I knew where it was all the time).