"Premature graying is genetically determined for the most part," David Bank, MD, tells WebMD. Bank is director of the Center for Dermatology, Cosmetic, and Laser Surgery in Mount Kisco, N.Y. "Graying is natural. We all do it eventually."
When more than half the hair is white by age 40, though, this is considered "premature."
Bank also points out that head hair grows more actively than body hair. So the head can go gray while other hairy parts remain your normal color. Correction: Your former color. Read more about the ABCs of premature graying hair.
Article: The ABCs of Premature Graying