What causes hair loss for teenagers?

Excessive amount of sebum on the scalp

Stress

For me it was always Hormones or when I stopped caring for hair and took the following steps:-

1. Stopped oiling for 1 year ( even though I had an oily scalp, my hair kind of became rough).
2. Lack of self-awareness for taking care of hair.
3. My current apartments provide 60% filtered water, so the rest harmful chemical combinations made my hair rough. ( I used to have silky hair before I moved to my new apartments because the previous one provided 100% filtered water).
4. Stereotypes not to let teens take care of themselves lol, else they'll become bad.

some hair loss I experienced myself was caused by treating flu with antibiotics

pretty much

stress might be a big factor

lack of sleep, stress and lack of basic hygiene

Cancer

Genetics

Alopecia