What is the reason for people who are only teenagers having grey hair?

i saw a couple of people in my grade 10 class having grey hair.





why is this?What is the reason for people who are only teenagers having grey hair?
Genetics. I started to get grey hairs at 16 and now at 36, I am about 10% grey. When I first saw the grey hair I panicked and thought that I would turn completely grey very quickly but it is a slow process thankfully! Both my parents started going grey very early so it is totally down to genes in this case.What is the reason for people who are only teenagers having grey hair?
hereditary. All in the genes.
not enough Yin and too much Yang