Recently a study was done following 3 million patients after they had kidneys stones. These patients were followed for an 11 year period. What they found out was exciting and concerning.
Hair cells, divide so often, that they can duplicate themselves in a few short hours. Some divide faster, that is why some hair grows quicker. As we get older, the cell division, for the most part, slows down. Only the cells of bone marrow grow quicker. Pigment changes results in hair color graying.