What should you do if you're 13 and your penis curves down at a 90-degree angle when erect?

i have never heard of that. i do think you should see a doctor I agree, see a doctor. There is a condition called Peyronie's disease which sounds similar although it is more common in older men. Have you ever fallen on it? Check out this website. http://www.urologychannel.com/peyronies/index.shtml