Genes & Development
The journal "Genes & Development" publishes research from the University of Texas Southwestern Medical Center in a mouse-model study (which may translate over to humans) alluding as to why hair turns gray and goes bald. The scientists describe a protein called KROX20 which switches on skin cells that become a hair shaft, which then causes cells to produce another protein called stem cell factor. In mice, these two proteins are important for baldness and graying.
Sources: Time