Why does hair turn gray?

Each and every hair arises from a root, which is surrounded by a special tissue mass called hair follicle.

Hair follicle is an elaborate structure present deep within skin epidemis. Inside follicle colour producing cells are present which provide colour to individual strand of hair. Pigment production gradually diminishes in ageing follicles. Hence some grey strands start arising. In old age, all strands tend to become white.