It's been known for years that hair turns gray due to a natural buildup of hydrogen peroxide in hair follicles, which causes oxidative stress and graying. (Hydrogen peroxide solutions have been used for years as a cheap and easy way to "go blonde.") [8 Tips for Healthy Aging]
In younger people, an enzyme called catalase breaks down hydrogen peroxide into water and oxygen. But lower levels of this enzyme, combined with lower levels of enzymes called MSR A and B that repair hydrogen peroxide damage, cause hair to turn gray as people age.