Gray hair at a young age?

Diana • Not TTC at the moment .
Hi guys 
I'm 20 years old and I noted a few gray hairs appearing on my head...: I wanted to know the reason for it and what I can do to prevent it? 
Maybe my diet or something? 
I just don't want grey hair at such a young age!!! 
Btw I don't smoke, drink or do anything to my hair to cause damage to it.