Dry, flaky skin can be caused by many different problems, including:
Cats often develop a greasy coat and dandruff if they are feeling poorly.
Cats tend to groom a little less once they get older, often due to arthritis and stiffness.
If you cat is in pain they will probably spend less time grooming.
Skin infections due to bacteria, yeasts or fungus (such as ringworm) can cause flaky skin.
Mites and lice
Parasites such as mites and lice usually cause very itchy, flaky skin.
Fleas cause itchy skin, scratching can lead to dandruff.
Allergic skin disease
Atopic dermatitis, food allergies and flea allergies can all cause itchy, dry, flaky skin.
Other diseases such as diabetes, Cushings disease or liver disease.