Grey hair happens when the melanin under the hair follicle stats to loose its pigmentation due to age. The age at which we start to develop grey hair is genetically determined, that is why some people develop Gerry hair when they are still young, in their 20’s , while others do so as very late ages, in their 70’s or been 70’s.
People get grey hair as the get older because the cells in their bodies including the cells that produce pigment for hair start to die. When the pigment producing cells die then all the hair will turn grey. I don’t know why the pigment cells in the skin don’t die like those in hair follicles. Why doesn’t our skin also turn grey as we grow old? I don’t know.