This happened when I was 12.
I hadn't started masturbating yet. A few months earlier a boy in my group at summer camp had been shown by his older brother how to jack off and he proudly showed all of us his newly found skills. I had been wondering about this since then but just hadn't put it together yet. I had a wet dream each week or so at that point.
Our parents were out for the evening and I had put my two younger sisters to bed and told them not to flush the toilet while I was in the shower. I went and took a shower. I didn't close the door to my bedroom quite all the way while I showered. We had a glass door on the shower. Also it was an older house and had big keyholes. Through the partly steamed up door I noticed the light in my bedroom go off and the door to my bedroom open slightly more as two eyes appeared at the crack, one above the other. Knowing I was being watched instantly gave me a solid erection. I decided that I would give them a good show. I made sure that enough water got splashed on the door over and over to keep the glass good and clear. I made sure to turn towards them as much as possible.
Then I soaped up. Washing my erection was a whole different thing from anything I had experienced before. It was wonderously intense. As I was washing and washing my erection the door kept inching open until there were two pairs of eyes. I of course 'didn't see' them. Then I decided to make them work for it a little. I finished my shower and stepped right out into the room five feet from their beedy little eyes with my cock at about their eye level. So they just pulled back a little into the darkness of my bedroom.
I dried off, combed my hair and all still with that incredible hardon, still aware of their attention nailed to my cock. At that point I decided that I was going to get comfortable in my room. As I went to open the door to my room I ignored all the noises they made getting out of my room and the door closed. I found that If I positioned myself carefully on the bed I could make quite visible through the keyhole. I turned all the lights on so it would be good and bright and arranged myself on the bed for good visibility.
I was so hard it hurt. Every now and then I felt a little squeeze inside somewhere and some big drips of clear slippery liquid started coming out of my penis. Instead of letting it drip all over I decided to rub it around my penis. It felt even better than the soap. Ohhhhhh did it feel wonderfull. I just sat there slowly rubbing the stuff around and around and around. I heard some noises and arguing outside the door as they each tried to be the one watching.
Suddenly one of them ended the arguement by carefully and slowly opening the door big enough for one eye. The hinge screaked a little and I just ignored it. I just kept my eyes shut, except when I fluttered a lid just enough to see what they were doing every now and then, and was slowly rubbing my penis and moaning a little. That little moaning turned to quite a lot of moaning and more rubbing. Over a few minutes they inched the door open until once again there were two sets of eyes. Suddenly something happened that I had never felt before while awake and I let out quite a screech. They turned and ran. I was amazed. That was a first, but not a last.