Alzheimer's Disease: This is the most common cause of dementia in adults and can present with memory loss, cognitive decline, and changes in behavior. Early-onset Alzheimer's disease, though less common, can occur in individuals under the age of 65.

Vascular Dementia: This is the second most common cause of dementia and is related to reduced blood flow to the brain, often as a result of strokes or small vessel disease. Risk factors include hypertension, diabetes, and smoking.
Frontotemporal Dementia (FTD): A group of diseases characterized by the progressive degeneration of the frontal and/or temporal lobes of the brain. It can present with changes in personality, behavior, and language, and is a common cause of early-onset dementia.
Lewy Body Dementia: Characterized by the presence of Lewy bodies in the brain, this condition can cause cognitive decline, hallucinations, and fluctuations in alertness and attention. It is another common cause of dementia that can present at a relatively early age.

Vitamin B12 Deficiency: A treatable condition that can cause cognitive decline, neuropathy, and other neurological symptoms. If missed, it can lead to irreversible neurological damage.
Thyroid Disorders: Both hypothyroidism and hyperthyroidism can cause cognitive symptoms that mimic dementia. These conditions are treatable and should not be overlooked.
Chronic Traumatic Encephalopathy (CTE): A condition associated with repeated head trauma, which can lead to cognitive, behavioral, and mood changes. Although more commonly associated with athletes, it's a diagnosis not to miss in anyone with a history of significant head injuries.
Normal Pressure Hydrocephalus (NPH): A condition characterized by the buildup of cerebrospinal fluid in the brain, leading to increased pressure. It can cause cognitive decline, gait disturbances, and urinary incontinence. It is treatable with the placement of a shunt.

Huntington's Disease: An autosomal dominant genetic disorder that causes progressive damage to the brain, leading to cognitive decline, motor dysfunction, and psychiatric symptoms. It typically presents in mid-life but can have an earlier onset.
Prion Diseases (e.g., Creutzfeldt-Jakob Disease): Rare, fatal brain disorders caused by prions, which are misfolded proteins. They can cause rapid cognitive decline and neurological deterioration.
Wilson's Disease: A genetic disorder that leads to copper accumulation in the brain and liver. It can cause neurological symptoms, psychiatric issues, and liver disease, and is typically diagnosed in younger individuals but can present at any age.
Inherited Forms of Dementia: Such as familial Alzheimer's disease, frontotemporal dementia, and others, which are caused by specific genetic mutations and can have an early onset.