What causes your face to get darker than your body?

AS your body ages, biological changes, both physical and chemical, can cause pigmentation changes. (Gain\Lose freckles, moles, liver spots, etc.) Those same changes can cause hair to lose pigmentation and alter pigmentation. An example is someone with light red hair when born and grows progressively darker as they reach adult hood. An additional example would be graying hair in elderly people. The changes are influenced by environment (diet, excercise, living location, climate) and genetics.