Hair goes gray when color-producing cells stop producing pigment, Naturally occurring hydrogen peroxide can also build up in the hair, bleaching the color. Typically, white people start going gray in their mid-30s, Asians in their late 30s, and African-Americans in their mid-40s. Half of all people have a significant amount of gray hair by the time they turn 50. Going gray, by itself, does not mean you have a medical problem, except in rare cases. Contrary to popular belief, stress has not been shown to cause gray hair. Scientists don’t know exactly why some people go gray early, but genes play a large role. Also, a vitamin B-12 deficiency or problems with your pituitary or thyroid gland can cause premature graying that’s reversible if the problem is corrected. Visit a trichologist for knowing the exact reason of ur premature hair greying and getting the right treatment .