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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.
Gray hair is caused by the gradual reduction of melanin production over time within the affected hair follicle. The melanocytes in the hair follicles produce less and less melanin, and the result is a loss of hair fiber color strength. What we call gray hair is not gray at all if you look at the individual hair fibers. Some hair fibers will contain some color while others are virtually white. What we describe as gray comes about from our perception of the overall scalp hair color. The contrast between the hair with more color and the white hair causes the appearance we call gray hair. Thus two people with gray hair, standing side by side, may have different shades of gray. Blondes are most likely to develop a completely white head of hair in old age because their hair fiber has a very low density of pigment in it to start with.
The time and speed of gray hair onset is due in part to genetics. Certainly early onset gray hair development can run in families. In my own family women are invariably affected by early and rapid onset gray hair while men have a more typical gray hair development. This difference in gray hair development in my family might suggest that at least some of the genes that promote early gray hair onset are located on the X chromosome. However, there are most likely many genes that determine the time and speed of gray hair development located on several human chromosomes. What genes determine gray hair is not known and much research needs to be done in this area. The environment may also potentially modify gray hair development but there is effectively no research on this subject.