Gray hair develops as melanocytes decrease in number, but exactly when those cells begin to wane differs for each individual. However, there are some general trends. According to the article, “graying typically begins in the mid-30s for Caucasians, the late-30s for Asians, and the mid-40s for Africans.”A 2018 study published in the Journal of Cosmetic Dermatology found that among 1,192 volunteers between ages 18 and 20, those with premature hair graying (PHG) were more likely to have a family history of PHG, have a genetic tendency to develop allergic diseases such as eczema or asthma, or be vegetarian.

To a lesser extent, they were also more likely to be overweight or report having higher levels of stress in their lives. Smoking has long been considered a factor in premature graying, according to a 2013 study that confirmed the association among Middle Eastern men and women living in Jordan. The study, published in the Indian Dermatology Online Journal, found that among 207 study participants, smokers were substantially more at risk of PHG than nonsmokers.Vitiligo occurs when cells that produce melanin die or stop functioning, resulting in a loss of skin color in patches over the body that usually grow over time. In some cases, it also causes hair to lose color. With alopecia areata, patches of hair — especially colored hairs — can be suddenly lost, which may result in more noticeable gray hair. Shmerling noted in a 2017 article that the result could explain why some people seem to turn gray overnight.