People in their thirties will find gray strands here and there, usually starting at the temples. But half of adults will lose 50 percent of their hair color by age 50. Going gray is hereditary; and some people go prematurely gray, where gray hair appears as early as in one’s mid-20s. Research on twins has found that they go gray at about the same time. And according to the National Institutes of Health, stress can cause premature gray hairs to set in; stress affects stem cells that regenerates hair pigment.