Pretty young. I was at a daycare and there was a closet that we would sneak into an pull down our pants. It wasnt sexual just curiosity, but hard non the less.
Thats when I discovered the difference between a circumsized penis and an uncut one, me having foreskin.

I was 11 I came home from school he was in our room naked stroking it, he taught me alot that day

I can't remember exactly, but I'm guessing it was at about 9 or 10, and it was my school friend who lived across the road. We used to strip off and play with eachothers' little stiffies. It was a crazy doctors and nurses game but just between two boys. The boy patient would see the male doctor to explain the problem which was always cock related.
The patient always had to get naked for the doctor to examine him, and the cure was always for the nurse to either suck the offending cock, or let the patient have sex with her.
The doctor would then change functions and become the female nurse who would strip naked as well, then either get on her knees to suck the patient or alternatively, bend forward over the straw bales to have the patient's cock rubbed against her "pussy" which was basically, just between the cheeks of "her" bum.
There was no cum at all but I assume we used to get a tickle out of it, otherwise, why would we keep doing it.
We got caught by his mum which put a hold on it for a while but not for long. Years later, we involved my sister which made it more fun.

When I was 13 I jerked off my friend. I loved holding his hard cock.

I have seen a couple hard ones in locker room.. I think once I got caught staring or he realized I was getting hard.

Apart from my own, it was a friend of my age - we were about 9 or 10 - and we were in the cinema in the front row on a Saturday afternoon, so the place was almost empty. He grabbed my hand and put it on his lap. I felt something soft and fleshy and realised it was his cock, not stiff at that point. I realised what it was and was a bit shocked and snatched my hand away. I became curious and after a minute or two I reached over to see if I could find his cock. He'd put it away. He took it out and there it was, standing up and illuminated by the light from the big screen. I gave it a good feel though had no idea quite what to do with it. I got my cock out and we sat there feeling each other, and I was converted to feeling the cocks of my friends and getting mine felt too.

Think I'd have been 11. I had a neighbour three or four years older than me who sometimes let me look at his porno mags when he was in his shed. One day he asked if I wanted to see him cum, so curious as I was I followed him up to his flat. He lay down on his bed and I watched him wank his hairy cock for a few minutes before he let go with some foamy looking white spunk. Cleaned himself off and that was that, kept it a secret. Wasn't long after that until I became able to cum myself and have been an avid wanker for some 45 years now.

I was 9/10 and was an older friend of the family. You never forget that kinda thing lol.

Always saw the parents nude. Probably 5 or 6 seeing it for the first time.

Technically: Mine
Other guy: Probably in the first porn movie I ever saw "Babylon Pink". (Or maybe it was called "Babylon Blue".)

I'm sure the first hard cock I seen was my own.Was around 9-10 at the time.I took control and haven't let go since.

Besides my own? Game of Truth or Dare when I was about 12. Saw my first live sex show that day, too. And performed a little myself. What else was there to do in a small Kansas town in the summer when you had nothing else to do?

My first was a school friend when I was about 12 and then various hard cocks of my mom's many boyfriends in my teens

I heard about the ymca in the past where there were nude swims. I love reading when the first time stories. We all are male and all grew up going through similar. Done boys had it easier than others but we all figured things out at our own pace. So great being about to celebrate and share with fellows bators here.