Your hair can and will change color when you are old, but this does not happen overnight.
Different hair colors, from blonde to black, are determined by the melanin, the coloring matter, in your hair cells. This melanin becomes part of your hair cells as they form in the roots. Gray hair starts to appear in older people when less melanin is deposited in the new hair cells as they form. Without this coloring matter, the hair becomes gray and then white.
Certain diseases can cause the body to stop or slow down the production of melanin. So might worry, shock, or deep sorrow.