I think that I am very qualified to answer this one! My hair started turning grey when I was 18. To be precise, you don’t turn grey, the hair is completely white. As long as there are many non-grey hairs, the mixture of white and non-white appears as grey, or ‘silver’, or ‘salt and pepper’ if you have mixed black and white hair. If you are blond, this effect is less pronounced and the greying hair looks a bit more yellowish. Your hair turns white because the cells from which the hair grows, stop producing a pigment called melanin.
You may have heard about an inherited disease called albinism in which the people have no pigment at all,and their skin is very pale and the hair completely white.
What I find interesting is that while your hair on the head turns grey (or white) usually your eyebrows and eyelashes keep their original colour. If anyone knows why, or how, please answer below!!!
Franco is on the money with this answer. Let me put an evolutionary spin on it. I would ask what is the point of our hair turning white?
The answer is probably to signify that we are no longer needed for the transmission of the genes in our bodies. Going grey is a sign of ageing (generally) and the fact that it happens when we are older possibly means it is protected in younger persons who are using their hair colour as a way of signifying their reproductive potential (eg , they are going to live long to nurture their offspring)
A bow to the amazing answers from Franco and Mark! 😀
On @Franco’s point for eyebrows and eyelashes retaining the original colour even when the hair turn white/grey, the internet didn’t give me any good answer (apart from how to do make up on eyebrows to look younger…)
So here is my own ***hypothesis***How about, the cells from which the eyebrows and the eyelashes grow are exposed to the sun more (because it’s on the face, while the skull hair tend to be longer and hence cover the hair-producing cells from the sun). With more exposure to the sun, this might stimulate the cells to produce more melanin (just like what happen with the skin cells) and hence the eyebrows and eyelashes remain coloured by melanin? This is purely a hypothesis that still need to be tested 🙂