It can be due to freckles or sun spots. Sun spots
or lentigines, are harmless, flat, brown discolorations of the skin
which usually occur on the back of the hands, neck and face of people older than 40 years of age. They are caused by the skin being exposed to the sun over many years and are a sigh of sun damage.
The only way to prevent age spots is to use liberal amounts of sunscreen before the age of forty, avoid the sun, and wear protective clothing.