Depression: Often presents with cognitive decline and can be accompanied by mild headaches due to tension or as a somatic symptom. Cognitive symptoms in depression can include difficulties with concentration, memory, and decision-making.

Chronic Fatigue Syndrome: Characterized by persistent fatigue, cognitive difficulties, and sometimes headaches, which could fit the presentation of mild headache and cognitive decline.
Sleep Disorders: Conditions like sleep apnea or insomnia can lead to cognitive impairment and headaches due to lack of quality sleep or disrupted sleep patterns.
Hypothyroidism: Can cause cognitive slowing, memory issues, and sometimes headaches, making it a plausible diagnosis for these symptoms.
Vitamin B12 Deficiency: Leads to neurological symptoms including cognitive decline, fatigue, and sometimes headaches, due to its crucial role in the nervous system.

Brain Tumors: Although rare, brain tumors can present with gradual cognitive decline and headaches, especially if the tumor increases intracranial pressure or directly affects brain areas responsible for cognition.
Chronic Subdural Hematoma: More common in the elderly, this condition can cause gradual cognitive decline and intermittent headaches due to a slow bleed in the brain.
Infectious Diseases (e.g., Neurosyphilis, HIV): Certain infections can directly affect the brain, leading to cognitive decline and other neurological symptoms, including headaches.
Vascular Dementia: Resulting from reduced blood flow to the brain, it can cause cognitive decline and sometimes headaches, especially if there are associated vascular headaches.

Creutzfeldt-Jakob Disease: A rare, degenerative, fatal brain disorder that leads to rapid cognitive decline and can be accompanied by headaches among other neurological symptoms.
Hashimoto's Encephalopathy: A rare condition associated with Hashimoto's thyroiditis, characterized by cognitive decline, seizures, and sometimes headaches.
Whipple's Disease: A systemic bacterial infection that can affect the brain, leading to cognitive decline, among other symptoms, and is often accompanied by gastrointestinal issues.
Mitochondrial Disorders: A group of disorders that affect the mitochondria, leading to a variety of symptoms including cognitive decline, headaches, and other neurological issues, depending on the specific disorder and areas of the body affected.