Let's start off by saying I was 17 at the time this happened and it was on Christmas Eve last year. I am about average height and weight for my age, but not particularly fit or athletic, and I have a 5.5' hard dick with a nice bush.
On Xmas Eve, my family went over to my cousins house for dinner. I've always wanted to teach someone to masturbate or even just to masturbate with someone my age or younger but I never got the chance. My cousin just turned 13 at the time and it about the same as me in terms if weight for his age. I've been thinking for a while that it would be awesome to jerk off with him since he's fairly cute but I've never seen him naked until that night.
After dinner, me and my cousin went up to his room to play some video games as we usually would. Because of being so busy with school and work lately I hadn't gotten the chance to jerk off in a few days as normally I would every night. Because of this, while playing video games I started to get a boner and was really horny. I started thinking about jerking off with him, what he would look like, how big he was, etc. and was getting even more horny.
I figured I'd just go for it and I asked him if he jerks off. I was so nervous already and still had a really hard boner. To my surprise, yet delight, he asked me what jerking off was. I figured this would make it so much easier to teach him and do it with him. I gave him the speech about what it is, what happens afterwards, privacy issues, and other related stuff. I was so horny to be talking about this with him and I was so nervous but I could see that he was too. in his jeans I could see he was also sporting a boner.
Since I knew we had a lot of time alone without being interrupted, I asked if he wanted to try. He thought about it for a second and hesitated but then said he'd like to try. I told him I'd show him how first so he can see how it's done, and with that I pulled down my jeans and boxers and got fully naked to reveal myself to him. I could see he was shocked to see my dick as I'm sure he hasn't seen another one before (though neither have I). I started slowly stroking my uncut dick but I was so horny and if I kept going for much longer I knew I could blow.
He took off his clothes to reveal his hard 4.5' dick with a small amount of pubes around his shaft. Even though I'm straight I still loved the sight of seeing him naked right in front of me. As I kept slowly stroking mine he tried to do the same but couldn't get the motion going well. Without asking I just took my hand and wrapped it around his dick and told him I'll help so that he can learn. he was shocked and nervous for a second but once I started stroking him he said it felt incredible. He was moaning quietly and started to leak precum. I kept going and started to go faster. I could tell he was getting close as he was breathing heavily and moaning lots. Once he said told me to stop and it felt like he was going to pee, that was my cue to keep going. Soon afterwards he shot out rope after rope of cum all onto his chest, belly, pubes, and my hand.
He told me how incredible that was and thanked me. I also thought it was incredible as it was my first time doing anything sexual with someone else. Without any indication, my cousin reached over and started to stroke my dick for me. I was in awe and was so hoping he would do this for me. He only had to do this for about 30 seconds before I felt it building up and I told him to keep going. I shot multiple ropes of cum as high as my chin and afterwards we bother were in a huge mess. We wiped up and joined our family. This experience made us a lot closer as cousins and we have jerked off many times since whenever we get a chance.