Why does hair turn gray?
To understand this, we should first answer what causes hair color to begin with. All hair, whether on your head, your arms, or your cat’s tail has pigment cells called melanocytes. These melanocytes produce a pigment called melanin- more specifically, the chemicals eumelanin and pheomelanin. Hair with more eumelanin will be darker (brownish to black) and hair with more pheomelanin will have a red/orange/yellow tint. Hair with less of either will be lighter.
These melanocytes also pass this pigment to cells called keratinocytes (which produce hair’s main component, the protein keratin). When the keratinocyte cells die, they keep the melanin which is what is visible and gives hair its color.
As we get older, the melanocytes get less active, thus producing less pigment and making your hair lighter. Eventually all the melanocyte cells die and there are none left to produce any color.
So what are the factors that control this production of pigment? Many are genetic. Alleles of the recently-discovered MC1R gene have been shown to produce red hair in mammals. Other genes, many as yet unidentified, are likely responsible for other hair colors. According to Laurence Meyer, a dermatologist at the University of Utah, “Generally speaking, among Caucasians 50 percent are 50 percent gray by age 50. There is, however, wide variation.” Of course, these percentages are different for different ethnic groups, which further emphasize the role genetics play in the color of your hair.
Scientific American: http://www.scientificamerican.com/article.cfm?id=why-does-hair-turn-gray