I was 13 then
By the time I was 13, I had some early naked play with a neighbor kid that was the same age as me but I really did not know anything about sex and neither did he.
My 14 year old cousin came to stay with us for a month. He lived in another state and although we visited every couple of years, I didn't know him very well. Because it was summer, my father decided that my cousin and I should sleep in our camper since there were not enough beds in our home to accomodate him. I was supposed to keep him company. The first night in the camper my cousin quizzed me about my friends, girl friends (none yet) and eventually our talk turned to sex. According to my cousin, he had a little experience with girls but nothing to brag about. At one point he asked me if I jacked off, and I told him I didn't know what he meant. He described jacking off and used the word cum and again I asked him explain what cum was.
He asked me if he wanted him to show me how to jack off and I said I did. Lying on the bed, we both removed our white briefs and turned on the light, only to discover we were both hard. Although I was entering puberty, my cock was still hairless and probably only about 5' hard. On the other hand, my cousin looked like a full grown man. He had thick blonde pubes and his cock looked huge compared to mine. However, probably because we were cousins, our cocks looked amazingly similar in shape, both circumsized. I stared at his enormous cock in awe and could feel butterflies in the pit of my stomach. Being naked and hard with my cousin suddenly felt very different from being naked and hard with my neighbor.
My cousin just laid back and explained the techique of jacking off. All I could do was stare at him expertly working on his shaft. After a while he pointed out his precum forming on the head of his cock and explained that it was called semen. He told me I could touch it if I wanted. I slowly moved my hand to his cock touched the clear substance with my finger. I was amazed that our cocks could produce something other than piss. He then asked me if I wanted to make him cum as he moved my hand to the base of his cock. I couldn't believe how thick and strong his cock felt in my fist as he instructed me on using the right amount of pressure and motion. I was very excited and was an eager learner. I suddenly felt all grown up.
In time he asked me to jack him faster and his legs and torso began twitching on the bed. Finally, he closed his eyes and begin gasping that he was about to cum. I could feel his cock begin throbbing in my hand as several ropes of semen shot out of the tip splashing his chest and also my hand. I was amazed at what had just happened because honestly, my friends had never discussed this simple fact of life. My cousin then told me he was going to jack me off and proceeded to gently wrap his hand around my throbbing boner, repeating what I had done to him. At first it just felt nice but in about ten minues of jacking me off I began to feel this incredible sensation unlike anything I had ever experienced.
Suddenly, I thought something had gone wrong and that I was going to piss but my cousin, sensing my fear, placed his other hand on my chest and somewhat held me down on the bed telling me to just relax. I can remember my mind alternating between feeling so much pleasure and so much fear I think I must have cried out as my toes curled and my cock began to spasm in my cousin's hand. However, to my surprise, it was a dry orgasm (as my cousin later explained) but all the same, it was the most incredible feeling I had experienced until then. The next day I introduced my cousin to my neighbor and the three of us spent many hours naked together learning about our cocks and other things that guys could do. Although my cousin and I never talked about that summer, I hope he reads this so I can thank him.