Why does hair turn gray as we age?

Hair turns gray when pigment-making cells in the follicle slow down or stop adding color to new hair. Without enough melanin, a strand grows pale, silver, or white instead of matching the color it had before.

It usually happens strand by strand, not all at once. Genes matter, and time matters too, as those color-making cells collect wear from years of work; the result is a changing mix of old color and bright new silver.