Moles occur when cells in the skin grow in a cluster instead of being spread throughout the skin. These cells are called melanocytes, and they make the pigment that gives skin its natural color. Moles may darken after exposure to the sun.
A few of these moles, called congenital melanonaevi, are usually present at birth. But most develop spontaneously or are caused by exposure to sunlight and tend to appear on those areas of the skin that catch the most sunlight. Most of these moles appear during the first 20 years of life, although they may continue to develop into the 30s and 40s.