Over time, the pigment-producing cells in each hair follicle tend to slow or shut down, turning hair gray, said Desmond Tobin, director of the Charles Institute of Dermatology at University College Dublin in Ireland. These strands tend to be stiffer and harder to control, growing 10 percent faster than pigmented hairs. Some people embrace them as a part of aging; others see them as a nuisance to be dyed, plucked or hidden.