Funny story, my Mom kept me very sheltered when I was growing up. When I graduated and went to college she was hysterical. She couldn’t believe that I was going to be on my own, I myself couldn’t believe it. It was during my freshman year that I learned about sex, where I lived with another woman my age who told me that all the things happening to my body were normal. It was embarrassing at first, to be so behind everyone else.
If you can guess what happened, my sophomore year was finally when my true adult self kicked in. I went to parties and had sex for the first time and I made a lot of childish jokes about the human body. I had to figure it out mostly all by myself and when my mom found out she panicked so bad that she almost didn’t let me go to school again. I promised I would never do that to my child. So yes, it’s very possible for someone to not know anything about sex or the natural functions of the human body until you turn 18. Especially if you had parents like mine.