system
Given the following post, respond as if you're commenting on Reddit:
human
Why do our hair colors change (greying/white hair) when we grow older or are stressed? 

I've always wondered about this phenomenon. It seems kind of odd that people grey as they grow older as hair does not seem, to require a lot of energy to maintain.

Can someone explain why this happens? And how does stress factor into people greying faster?
gpt
According to a recent article in the Federation of American Societies for Experimental Biology (sorry, no link, subscription site):

Oddly enough, your follicles produce three things simultaneously: melanin, which gives your hair its natural colour, hydrogen peroxide (as part of the follicle's normal oxygen intake cycle), and the enzyme catalase, which breaks down said peroxide into hydrogen and water almost as soon as it forms. As you age, your catalase production decreases, leaving more peroxide hanging around in the follicle, which bleaches out the melanin colouring in your hair proteins. Your keratin turns to its neutral-state colour, which is a semitranslucent silvery-gray.

And btw, psychological stress doesn't "cause" gray hair. Stress can sometimes (rarely) cause *alopecia*, which means the thicker, darker hair can stop growing, leaving the already-gray remainder more prominent, but it won't cause the hair to actually change colour, per se.  Some health issues, like insufficient B12 intake, smoking or thyroid problems may accelerate the graying process. Genetic predispositions also come into play, which is why caucasians generally tend to go gray earlier and more completely than, say, Asians.