When I was 15 years old my eight-year-old cousin used to visit often. While visiting, he was in the habit of hiding in my room and sometimes popping out to scare me. Other times he would stay in my closet until he couldn't help laughing and then I would find him. My Mother and Aunt were well aware that my cousin was always sneaking into my room and hiding, they also knew that it got on my nerves.
One day my Aunt assured me that my cousin would never hide in my room again, but of course a few days later he popped out of my closet and scared me. No matter how many times they talked to him he would keep doing it, until the point that I got a lock put on my door. The only rule for the lock was that I left a key hanging in the kitchen. One day I knew that my cousin had got the key and sneaked into my room. I could tell he was in the room because I could hear him breathing and giggling.
I was horny and had a boner in my pants, having planned to jack off. The prospect of having somebody watching only made me want to do it more. I reasoned that I couldn't be blamed if he told my Mother or Aunt, because I had done everything I could to keep him out and how was I supposed to know that he had the key? Knowing full well that my cousin was watching me I dropped my pants and took off my shirt. As soon as I had done this the giggling stopped.
My dick was already its full size, which is about six and a half inches and uncircumcised. I had seen my cousin naked before and new that he had a stubby little circumcised dick, so I thought about how different mines must have looked. My door was locked and I was fully nude with my cousin peeking out of the closet. I jacked off for 15 minutes and he didn't say anything. When it was time for me to cum, I did so making sure he could see everything.
I then opened the closet, planning to act angry and shoo him out of the room, but when I saw him he had a whole bunch of questions for me. I spent the next 20 minutes talking to him about what he had just seen and assuring him that with the exception of the foreskin, his stuff would look the same as mine eventually. I showed him the t-shirt I had used to catch the cum and explained it all came from my balls. I said, 'when you get older not only do you have to pee and poop, but you have to cum.'
After that, my cousin never sneaked into my room again, but always addressed any questions about his privates to me. When he was 11 years old my Aunt wanted me to have the birds and bees discussion with him, because he didn't have a dad and she felt that a guy should talk about it with him. What she didn't know was that we had already been having an ongoing discussion for three years.