From an online article entitled "Gray hair and age":
The first onset of gray hair and the speed at which people go gray varies considerably from person to person. Most people actually start going gray in their late 20s but they don't notice it immediately. Premature graying is defined as gray hair onset before late teens for Caucasians and before age 30 in Africans and Asians, or alternatively 50% or more gray scalp hair before age 50. Very occasionally, a few gray hairs can develop in children as young as 8 years and yet it indicates nothing other than an early onset of the gray hair that we all develop with increasing age. Typical gray hair first develops at age 34.2 +/- 9.6 years in Caucasians while for Black people the average age of onset is 43.9 +/-10.3 years (Keogh 1965). As a rough guide, 50% of the population in the US and Europe have 50% gray hair by age 50.
The most common areas on the scalp in which to first see gray hair development are above the ears and/or at the temples. This first gray hair may spread around the sides and to the crown with time. Gray hair development in the beard and mustache may also start quite early, while gray hair on the chest and pubic region generally only occurs some years after onset of gray hair on the scalp.
I just found my first gray chest hair.
I don't know why this disturbs and distresses me in a way that the gray at my temples and liberally salting my goatee has not.
But it does.
I had my first gray around ten.
I remember finding a few strands of gray around that same age too, though it went away and I didn't see any again until some time in my mid-thirties.