How do I grow body hair?

(I'm 18 now) When I was 13 I just stopped growing body hair. My upper thigh hair is like a millimeter long and I have lots of black-spots (Where hair is suppose to be), my eyebrows are patchy and my arm hair has actually thinned out. I have three, nearly invisible chin hairs of a beard.

Pubes and headhair grow as normal.