There is pigment found in the skin as well as the hair is referred melanin that is responsible for colour production. These melanin can be categorized into two forms: eumelanin (deals with the production of brown or black colour) and  pheomelanin. As most people age, the begin to lose the melanin in their hair while some are able to retain small amount of these melanin. Grey hair is fundamentally hair with small amount of melanin, whereas white hair lacks melanin in totality. However, the control of genes play a very vital role in such cases. For some families, their members begin to grow grey hair at very young age- I even have friends of my age with grey hairs. The exact reason why the hair is able to lose these melanins is still unclear to many.