Melasma: This condition is characterized by patches of discoloration, typically on the face, and is common in women, especially during hormonal changes. The mustache area is a common location for melasma, and its prevalence increases with age and sun exposure.

Hirsutism: Although more commonly associated with excess hair growth, hirsutism can also lead to skin darkening in the affected areas due to increased melanin production. In an 80-year-old female, this could be due to hormonal imbalances.
Contact Dermatitis: Irritant or allergic reactions to substances like makeup, skincare products, or even certain metals in jewelry can cause skin darkening and discoloration in the mustache area.
Post-Inflammatory Hyperpigmentation (PIH): Following any form of skin injury, including acne, dermatitis, or even minor trauma, there can be an increase in melanin production, leading to dark spots or patches.

Addison's Disease: Although rare, this condition, characterized by adrenal insufficiency, can cause generalized skin darkening, including the face. It's crucial to consider this diagnosis due to its potential severity and the need for prompt treatment.
Cancer (e.g., Melanoma): Any new or changing skin lesion, especially in an elderly individual, warrants careful examination to rule out malignancy. While less common, skin cancers can present with discoloration.

Erythema Dyschromicum Perstans: A rare condition characterized by the gradual onset of blue-gray patches, typically on the face, neck, and upper extremities. It's more common in younger individuals but can occur at any age.
Argyria: Caused by exposure to silver or silver compounds, argyria leads to a blue-gray discoloration of the skin and mucous membranes. This is extremely rare and usually associated with occupational exposure or the use of certain medications.