Why Gray Hair Grows
It's hard to imagine that those lovely locks of blond or brunette hair on your head may someday change color so dramatically. But your hair doesn't turn white from a lifetime of watching really scary horror movies. Instead, it's actually revealing its natural beauty. Hair loses much of its color because it's losing its melanin or carotene, which are your body's natural coloring substances (also known as pigments). As you grow older, the pigment production in hair follicles slows down and may eventually stop, cutting off your hair's supply of coloring and making it appear gray or white. But your entire head of hair won't turn gray overnight; after all, this is a process that takes about 10-20 years.
So when exactly will your hair start turning gray? It'll probably depend on your own biological time-clock, as set by the genes you've inherited from your parents, your parents' parents, and beyond. And that explains why some people get gray hair while they're still in their teens and twenties--the melanin-producing cells of those people were set to stop working much sooner than others. But if this happens to you, don't fret over your follicles. There's nothing wrong with showing maturity at an earlier age!