All hairs on the body, as well as on the head, are produced by the hair follicles at the base of the hairs. Even if a hair falls out, as long as the hair root is alive, a new hair will grow right back. However, if the cells that make the hair follicles die, the hairs will never grow back. However, it is still not well understood why these cells die.
It is said that the reason for hair loss in old age is that the skin of the head is not soft enough and the skin crushes the blood vessels, preventing oxygen and nutrients from being supplied to the head.