Is there a cure for dementia? Is dementia reversible?  

Short answer is no. Currently the major types of dementia: alzheimers disease, vascular, frontotemporal and lewy body are not preventable. Exercise has a protective effect against alzheimers though it likely delays it rather than prevents it altogether. All those things that are good for your heart are good for your brain: maintain a healthy diet and weight, exercise, manage high blood pressure, diabetes etc.
No "Cure" But, other diseases or illnesses that can cause cognitive impairment are sometimes treatable. These include hypothyroidism, depressive pseudodementia, tumors, normal pressure hydrocephalus, and deficiencies in vitamins b1, b12, and a.