An English - German team of scientists have found the answer to the mystery:
Why our hair turns gray?
“In essence, the researchers found that the body literally bleaches its own hair. Hair goes gray because there is an accumulation of hydrogen peroxide caused by the wear and tear of the hair follicles. All hair cells produce hydrogen peroxide, and as we age, the amount increases. The peroxide blocks the normal production of melanin, which is the hair’s natural pigment.”