I think I haven't fully physically matured yet ,how long would it take to gain 1"?

Endocrine system was one of my weakest areas. I actually had a very difficult time with endocrine diseases. Kidney and heart where my favorite. Much simpler organisms.

However, with a hormone like testosterone, most people as they age the receptors are still there it is the testosterone levels that are dropping in the blood.

I truly don’t know which hormone would have a positive effect on penis growth. My guess and just a guess would be multiple hormones and they would probably be more about erection quality.

I agree that more erections would probably be better. Just having good blood flow through the penis is probably helpful.