No known agent for preventing dementia, or general cognitive decline, has ever been identified, except for. . . alcohol.
Sorry. We're not allowed to know this — you won't find it trumpeted by government and other health agencies, that's for sure! A comprehensive meta-study published in Age and Ageing in 2008 — one combining the results of more than 20 studies selected for methodological soundness, and that thus has a firmer foundation than any individual result — found a reduction of 35-45 percent in the risk of cognitive decline or dementia among regular light-moderate drinkers currently age 65 or older.
This effect is not as decisive as alcohol's powerful role in preventing cardiovascular disease (and thus death, since heart disease is our leading killer), which we're sort of, almost allowed to know about. An analysis in the current issue of Psychiatry Investigation, "Alcohol and Cognition in the Elderly: A Review," notes the evidence of this cognitive benefit, but finds it is less well-grounded than that for alcohol's role in reducing heart disease.