What causes hair to turn gray at an older age?

As we get older, the pigment cells in our hair follicles gradually die. When there are fewer pigment cells in a hair follicle, that strand for hair will no longer contain as much melanin and will become a more transparent color, like gray, silver, and white, as it grows. As people continue to get older, fewer pigment cells will be around to produce melanin. Eventually, the hair will look completely gray.