Generally, race seems to play a role in premature graying. Hair begins to lose color as early as the age of 20 for caucasians, 25 for Asians, and 30 for Africans. This relates to the observation that the lighter the skin color, the sooner that hair will turn gray. Graying can be genetic so the exact age when it starts is individual.
At the root of this aging process are hair cells called melanocytes that produce a dark pigment called melanin, which comes from the Greek word “melanos,” meaning “dark.” This pigment is what gives color to hair, skin, and the iris of the eyes. Hair color would depend on two groups of melanin. Eumelanin contains brown and black pigments while pheomelanin have red and yellow pigments. Variations in the amount and ratio of each result in a variety of colors and shades.
As we age, our melanocytes produce less melanin, causing our hair fibers to lose their original color. Aging hair, as professor Trueb calls it, is not just a matter of deteriorating hair color but of decreasing production and declining appearance as well.
Relative scalp coverage, which measures the average number of hair fibers in a cross-section multiplied by the number of fibers per square centimeter, usually peaks at age 35. Hair density peaks in the late 20s, but hair diameter can increase until age 45, after which hair growth slows down and fibers grow thinner and coarser. This is why gray hair is more resistant to artificial color.v
Aging hair is also more sensitive to ultraviolet rays, which cause hair proteins such as keratin to break down and make hair more prone to breakage. The production of sebum, the oily substance secreted by the sebaceous glands through ducts that open to the hair follicles, also decreases after age 50. The increase of sebum starting from puberty until it peaks from ages 45-50 is what keeps the hair moisturized, shiny, and soft. Declining amounts in older ages translates to hair that lacks luster and is coarse and dry.