Causes can range from just natural aging, to endocrine/metabolic abnormalities. You may have to get a couple of blood panels, along with scraping/biopsy to be sent off to a dermatologist.

If everything comes back negative, try supplementing the diet with fish oil. Probably 2000-3000 mg 2X daily might help. There's also an outside chance that a bizzare allergy might be involved, in which case Benedryl might help. But have the tests run first.