With aging, the number of melanocytes in the skin reduces. The melanocytes that are left undergo a compensatory increase in size and they then to focus in specific areas. This process is responsible for the development of age spots in people over the age of 40.
Injuries of the skin
After injuries to the skin which includes surgical incisions, burns, chemical injuries. The skin undergoes a darkening due to stimulation of melanocytes in response to injury. The skin develops post-inflammatory hyperpigmentation as a result.
Certain medications can cause hyperpigmentation including antibiotics, antidepressants, chemotherapy drugs, anti-epileptic drugs. Medical conditions that cause hyperpigmentation include Addison’s disease and Diabetes. Vitamin deficiencies can be a cause of hyperpigmentation.