It all depends in the cause of dementia.
Huntington’s disease that causes dementia is inherited, so this cause of dementia is hereditary.
With Alzheimer’s disease – the commonest cause of dementia – most cases are not inherited. In the rare cases when Alzheimer’s disease is inherited, generally speaking in these circumstances the disease develops at a much younger age, usually under the age of 65, often in a person’s 30s and 40s.
Having Alzheimer’s disease in the family does increase a person’s risk but only by a very small amount. Keep in mind that this doesn’t mean it’s inevitable that someone will develop the disease themselves.
A healthy lifestyle can help reduce risk factors that harm the circulation, such as smoking, high blood pressure, and high cholesterol levels that increase the risk of dementia, particularly of vascular dementia.