What causes the hair to grey?

As you age the follicles lose their pigmentation. Hair isn't actually grey its white, but it looks grey

Why hair turns gray as we agePremature graying can happen at any age ... generally from the 20's up. I knew a young man in Elementary school who had a tin can thrown at him which cut the scalp and he had a patch of white hair about two inches in diameter. The cut had caused damage to the hair follicles.

Gray hair can be hereditary and some young people may be what they call, 'salt and pepper'. It's genetics and it's no different than a 20 or 30 year old male going almost bald. These hereditary factors generally come from the mother's genes when it comes to hair loss.

Not all people that are getting older get gray hair. I am 65 and don't have one strand of gray hair (my mother didn't either) and my father had what I refer to as 'silver wings' just gray hair on the side of his head.

Once hair is gray it becomes coarse and it's 'dead hair' and there is no product out there that will reverse this short of dying. Gray hair also absorbs dye very quickly so if a person has black hair they are better to get it done professionally.

It is true as the above poster says that there are certain medical treatments and disease where the hair may revert back to color (and it won't be exactly the same color as they had before; perhaps a shade or two lighter.)

As we get older, pigment cells in our hair follicles gradually die. When there are fewer pigment cells in a hair follicle, that strand of hair will no longer contain as much melanin and will become a transparent color like gray, silver or white. When people get older, fewer cells will be around to produce melanin. The age which you go gray (or white) is determined by your genes.
There is something that colors your skin and hair called melanin. when your hair turns gray, it's because your melanin is decreasing in your body. It comes with old age.