Premature greying of hair starts when color-producing cells stop producing pigment. Naturally passing hydrogen peroxide can also make up in the hair, dulling the color.
Generally, white people start going gray in their mid-30s, Asians in their late 30s, and African-Americans in their mid-40s. Half of all people have a significant quantity of argentine hair by the time they turn 50.
A white person is considered to be precociously argentine if their hair turns argentine by age 20; gray before 30 is early for African-Americans.