What causes gray hairs to sprout as a person grows older?
Cells in hair follicles called melanocytes generate melanin and pheomelanin pigments. When the melanocytes stop producing these pigments, the hairs that sprout become transparent and appear gray or dull yellow.
The age at which people go gray seems to be hereditary, but gray hair can also be caused by a B-12 deficiency, a thyroid imbalance, or anemia. In addition, smokers are more likely to gray prematurely than non-smokers.
There is no consensus in the scientific community about trauma or stress causing people to go gray.
—The Fact Monster