Can the colour of skin change with age? Can a dark skinned person become fair or vice versa? Can Melanocyte production vary?
There are all sorts of things that can stimulate melanocytes, most notably UV rays (ie from the Sun). So yes, melanogenesis can very over age and exposure to the Sun. Young infants often are lacking build ups of melanin in their skin, eyes, and hair, which is why these things darken with age.
There are other more pathogenic ways to change melanogenesis, including malnutrition, but UV exposure this health common way.
Vitiligo is a loss of skin pigmentation that causes patches of lighter skin as the immune system attacks melanocytes. It can eventually progress to cover the entire body.
Ongenae, K., Van Geel, N. and Naeyaert, J.-M. (2003), Evidence for an Autoimmune Pathogenesis of Vitiligo. Pigment Cell Research, 16: 90–100. doi: 10.1034/j.1600-0749.2003.00023.x