The most common cause of dementia is
Causes of Dementia: 

Alzheimer’s  (most common in >65 years old)
Vascular dementia/Multi-infarct dementia
Lewy body dementia
Fronto-Temporal dementia (50-70 years)
Huntington’s
Traumatic/Dementia pugilistica
Creutzfeldt-Jakob disease

 
Reversible causes of Dementia

Structural brain lesions:- Normal Pressure Hydrocephalus, SubDural Hematoma, Brain Tumors
Alcohol dementia/ Wernicke’s encephalopathy
Vitamin B12 deficiency
Endocrine:- Hypo/Hyper-thyroidism, Hypo-Parathyroidism
Metabolic disorders:- Dyselectrolytemia (Na, Ca, K), hepatic or renal failure
Autoimmune:- Hashimoto’s paraneoplastic encephalopathy (anti- Yo, Hu, Ri)
Infections:- Whipple’s, Lyme disease, Syphilis, HIV dementia.
Fronto-Temporal dementia
Senile dementia
Multi-infarct dementia
Alzheimer’s disease