I was a relatively "late bloomer". However, I had a cousin who lived in another state who was my age in years but about two years ahead of me in his pubertal development. He introduced me to pleasuring myself and got me started.
My cousin, Chase, was visiting from the Midwest as he usually did every summer. He, his brother and mother would spend several weeks visiting family. During this time we would usually have several sleepovers. Since we were little, sharing a bed and even sharing the bathroom during shower time was no big deal. We had seen each other naked many times before, but this summer things had changed. Chase had developed! Looking back, I now realized that he was actually the more atypical one. He was probably a year or two ahead of your typical 13 year old boy. I, on the other hand, was a bit later than usual with my development, but that did not stop him from teaching me a thing or two.
During one of our sleepovers, we were sent upstairs to shower as usual and we both went in together. In years past, we would take turns in the shower. This year, as Chase began to undress, he suggested that I join him in the shower. At this point, mind you, my thoughts were nowhere similar to his. I agreed thinking it would save time and we could do other things later - I just had no idea what those other things would be. As Chase stripped down, I was in shock. His dick was huge with lots of thick, dark hair around it. I'm pretty sure he was already semi-hard, but it looked to be over 4 inches and soft. Now, I was very reluctant to strip, but he assured me it was OK and not to worry if mine wasn't as big yet. So, nervously, I removed my shorts and underwear. I was so nervous about our differences, I was anything but hard. Instead, I was probably suffering more from shrinkage at the moment. Anyway, we both entered the shower and goofed off for awhile and the shower probably took longer than if we had just showered separately. However, by the end I was totally relaxed being naked in front of him and I appreciated him not making fun my by smaller, early pubescent member.
Later, after all the adults had long been asleep, we were still awake talking and he asked me if I had ever looked at a Playboy. I had no idea, really, what he was talking about. Anyway, he said he knew where our uncle had some stashed away and he disappeared for about 30 seconds and returned with a few magazines filled with naked women. I really didn't know what to say and certainly didn't know what to do. I began flipping through the pages of one while he looked at another. Suddenly, I noticed him rubbing himself through his PJ shorts and that his cock was tenting them up. He told me that he would show me how to "use" these magazines if I wanted. At this point, I was in deep, so I said sure, why not. He instructed me to pull down my pants and rub my cock as I looked at the pictures and he would do the same. When he removed his shorts, I was, again amazed by the size of his cock. It was about 7 inches and thick! At this point, I was hard too, but only measured about 4 inches. He showed me how to stroke up and down on my shaft. He was lucky enough to use his whole hand, but, of course, that wasn't possible for me, so I just used a few fingers. No matter, I had never felt anything like this before. We lay there for about 10 - 15 minutes rubbing our stiff members when suddenly he began moaning and grunting. I looked over and there was a small pool of this white stuff on his belly. He explained to me that this was called "cum" and that I probably wouldn't make any yet, but that one day I would too. He asked me to get him some tissues to clean up which I did. I was still rock hard from watching him, and returned to stroking my shaft. He urged me to keep going and it would feel really good. I did and he was so right. Eventually, I felt a bold of electricity going through my body. I felt almost dizzy for a moment and shuddered in pleasure. I looked down, nothing. No cum for me, not this time. My cock was red and a little sore. Unfortunately, he did not teach me about any lubricant - I would figure that out for myself later!
We repeated this several more times that summer and summers in the future. As I progressed with my puberty, of course I eventually started producing jizz myself. We never actually touched one another, but would watch each other as we wanked. I'm pretty sure he was just as turned on by me as I was by him. I totally credit Chase for teaching me about masturbation and, I guess, owe him some thanks for all his knowledge.