Hi i discovered about 5 grey hairs in my head, i'm 19. my parents did not grey early. does this mean i have white hair by 20s. does stress cause this?

Yes and no: Severe illness and stress can indeed induce some hair changes, both in color and density. This is relatively uncommon, however. Just don't pull them out because 5 new will grow for every one you pull ;-)

No, probably somewha: You're a little early, but lots of people get some gray in their 20's and 30's and it doesn't progress much for decades. Stress probably contributes in extreme situations, just look at before and after pictures of presidents, but I doubt that's been scientifically proven.

Pre-mature graying: It is believed that hair turning white is caused by the production of hydrogen peroxide in the hair follicle. While stress can do many many things, there has been no direct linkage that I know of correlating one with the other.