When I was 12 years old, I was visiting my grandparents in Arizona, and one of their neighbors was a 9 year old girl named Jessica. Jessica broke her arm and had to wear a cast for 2 months. When I was visiting, she just had the cast removed, and her arm hair was significantly darker and longer than before. The longer darker hair was just on the one arm that had the cast on it. Before she had the cast, the arm hair was short, light, and barely noticble. She didn't have long dark hair anywhere else on her body, so it wasn't a hormomal thing. One of the other neighbors, Marge, who's a 50 year old something lady, said when she had a cast on her leg years ago and had it removed after several months, her leg hair was long and black which was unlike what it was before.
I've come to the conclusion that bad hygiene and lack of proper respiration for the skin can lead to induced hair growth. I'm not saying bad hygiene and lack of respiration for the skin are the only factors to hair growth; I know hormones have to do with hair growth too. I just think taking care of your skin is a big deal when it comes to hair growth, especially when you're young.