From ScienceNews: "Colorful locks depend on a group of special cells in hair follicles called melanocyte stem cells. Each of these cells divides into two cells: One that replaces itself and another that differentiates into a pigment-producing daughter cell called a melanocyte, which imbues hair with its browns, reds and blacks. Earlier research has suggested that the depletion of these stem cells was to blame for grayness. But how exactly these stem cells disappeared was mysterious. With no more stem cells around to produce melanocytes, hair turns gray. ... DNA damage causes them to lose their 'stemness,' the new report shows. Once the cells have racked up enough DNA damage, they become melanocytes and lose the ability to replace themselves or to replenish melanocyte cell populations. Once the melanocytes die, the hair is left with no pigment-producing cells." This should fuel the debate over the contribution of stochastic DNA damage to aging - perhaps diminishing stem cell populations is the important mechanism there. You might also compare this view of gray hair with the evidence for buildup of hydrogen peroxide as a cause.

Link: http://www.sciencenews.org/view/generic/id/44610/title/Stressed-out_DNA_turns_mousy_brown_hair_gray