If you started out as the blondest kid in your kindergarten class and became a brunette by high school, you’re not the only one. As people get older, their hair often gets darker.
According to IFLScience, this is due to changes in the production of melanin—the natural pigments responsible for hair, eye, and skin color. Two types of melanin are common: Eumelanin determines how dark your hair is, while pheomelanin controls how warm it is. In other words, black-haired people produce the most eumelanin, and red-haired people produce the most pheomelanin. Melanocytes are the cells located at the bottom of each hair follicle that actually create those pigments, and they take their cues from your genes.
But genes don’t direct melanocytes to produce melanin with the same consistency for your entire life—hormones can activate or deactivate certain genes. When you hit puberty, for example, previously latent genes may leap into action and create much more eumelanin than before, darkening your locks.
With time, however, cells aren’t able to regenerate as quickly as they can when we’re young, so melanocytes don’t churn out as much color. As melanin decreases, new hair comes in gray or even white. When exactly that happens to you mostly depends, again, on your genes—if every person in your extended family has dark tresses well into their sixties, your own chances are pretty good.
Does Stress Cause Gray Hair?
It is possible that major stress can speed up the process of going gray, but it’s not necessarily because your melanocytes stop producing melanin. Stress—brought on by physical or psychological trauma, serious illness, extreme changes in diet or weight, hormonal fluctuations, etc.—can cause a condition called telogen effluvium, where your hair falls out as much as three times faster than usual. For middle-aged people, the new hair that replaces it might be gray, giving the impression that stress was the direct cause.
That said, a 2020 study found that the fur of some mice exposed to stress turned gray for a different reason. When the mice were stressed—from exposure to a variation on capsaicin, which makes chili peppers hot—their sympathetic nerve cells produced noradrenaline. That, in turn, rapidly activated melanocytes and caused them to move away from their hair follicles. Without those melanocytes, the mice’s new fur grew in gray. In order to see if humans respond the same way, more study is needed.