I may have been 13 years old, I am now 66.
I grew up on a farm in the midwest, my only sibling was my sister, who is four years older than me, and my parents were very religious. As a result, I lived a very sheltered life while entering puberty. Sometimes I noticed waking up with a large damp spot on the front of my pajamas and other times, almost every morning, I would wake up and my noodle would be hard and stiff. Usually after I went to the bathroom the stiffness would go away, so I did not think that much about it.
One evening I was taking a bath and had really lathered up the soap on my chest and stomach before the water level had reached my noodle. For some reason I began to look at my noodle closely with my soapy hand. I began to feel a strange sensation, a kind of tingling or light burning inside my noodle, and it began to grow. As I touched my noodle with my fingers it was getting harder and harder and soon it was as hard when I would wake up in the morning. I placed my fingers entirely around my noodle and could feel my heart beating in it with my hand and I could feel it jerking in my hand. I gave my noodle two or three firm strokes and the deepest sensation rocked my entire body and something shot out of my pee hole onto my chest and face. I could feel that the stuff that shot out was warm on my cheeks and chin. I was shocked and just sat in the tub wondering if everything was ok with me. I soon realized I was ok; but, didn't know what that was all about.
The next morning my sister and I were in the barn doing our before school chores and I hesitantly asked about what had happened to me while taking my bath. She became very distant to me and just said that was what happens with boys when they get to my age and that every thing was normal and I would soon figure out what it was that was happening. That day at school I sat in class and thought about what had happened in the bathtub and what my sister had said to me. As I thought about it I could feel my noodle getting hard again and was becoming afraid that I was going to shoot that stuff out of my pee hole again. Thankfully, school was over and I rode home on the bus still wondering what had happened to me.
The next day must have been Saturday as my sister and I were alone in the house and my parents had left a note on the kitchen table that they would be gone most of the day doing their weekly shopping and business in town. As I was eating my cereal my sister came into the kitchen carring a book that she handed me and told me that the open chapter should explain to me what was happening to me and what it meant. She also told me that after I had read the chapter and looked at the pictures she would help explain it if I had questions after I was finished reading it. She smiled and told me to go to my bedroom to read it and to do it before I got dressed.
The chapter was full of medical terms and in a clincal lanquage. But, I figured out that my noodle was a penis and my siter's 'pooty' was a vagina and that put together with the warm stuff from my pee hole a baby could be made. It was a lot for me to understand, I was a little puzzled and all the time fully erect.
After about 30 minutes of reading and looking at my penis my sister knocks on my door and asks if she could come in. After I had covered up my erection I told her she could come in. When she entered she was smiling and asked if I had more questions. She knew that I had a lot of questions and said that she would explain everything to me as she knew my parents wouldn't. She locked my bedroom door, walked to the side of my bed and took off her night gown and stood there in front of me, letting me see her full naked body. She then got under the covers with me and told me to relax, to breathe deeply, as I felt the warmth of her body next to me.
She told me not to be embarrassed and to let nature take control of me and that it would soon be over she thought rather quickly. Not knowing what she meant, I soon found out, as her soft skinned fingers encircled my penis. With six or seven firm strokes I ejaculated onto my chest and face for the second time as I felt like I almost fainted from joy and pleasure. She smiled and hugged me telling me to lay there and relax as she cleaned me up with my underwear that I had taken off. After a few minutes she said that this was the only time this would ever happen between us: but, that she just wanted to help me as she knew I was frustrated about what was happening to me.
My wife died when I was 51 and a few years later my sister's husband left her for a 35 year old. My sister and I have been living together for the past 12 years and she was wrong, it has happened a lot since we moved in with each other.