Why Our Hair Turns Gray When We Gets Old?

That is because,

Hair turns gray as a result of a chemical chain reaction that causes hair to bleach itself from the inside out.

The process starts when there is a dip in levels of an enzyme called catalase. That catalase shortfall means that the hydrogen peroxide that naturally occurs in hair can't be broken down. So hydrogen peroxide builds up in the hair, and because other enzymes that would repair hydrogen peroxide's damage are also in short supply, the hair goes gray.