All the hairs on our head contain pigment cells that contain melanin. Pigment cells in our hair follicles gradually die as we age. The decrease in melanin causes the hair to become a more transparent color like gray, silver, or white.

Premature gray hair is hereditary, but it has also been associated with smoking and vitamin deficiencies. Early onset of gray hair (from birth to puberty) can be associated with medical syndromes including dyslexia.

-from Why Do Men Have Nipples? by Mark Leyner and Billy Goldberg, MD