Age spots are caused primarily by years of exposure to ultraviolet (UV) light from the sun. The use of commercial tanning lamps and tanning beds can also contribute to the development of age spots.
The pigment in the upper layer of skin (epidermis) that gives your skin its normal color is called melanin. UV light accelerates the production of melanin, creating a tan that helps protect deeper layers of skin from UV rays.
On areas of the skin that have years of frequent and prolonged sun exposure, age spots appear when melanin becomes "clumped" or is produced in particularly high concentrations.