Hair changes colour naturally because the pigments are no longer produced in the hair root.
Hair colour is produced by cells known as melanocytes, which migrate into the hair bulb as the hair follicles develop.
The melanocytes produce pigments called melanin that are incorporated into the growing hair fibres.
At the end of each hair cycle, some pigment-producing melanocytes become damaged and die.
If the melanocyte stem cell reservoir at the top of the hair follicle can replenish the bulb, this keeps pigment production going.
But when the reservoir of stem cells is exhausted, pigment production stops and the hair turns grey.
Graying of hair may also be triggered by a natural buildup of hydrogen peroxide in hair follicles.
Hydrogen peroxide is an oxidizing agent that often converts coloured compounds into colourless ones.
An enzyme called catalase breaks down hydrogen peroxide into water and oxygen.
But as people age, lower levels of this enzyme, combined with lower levels of the enzymes that repair hydrogen peroxide damage, cause hair to turn gray.