Dr. Demento?

Sez the BBC:

Exercising for half an hour at least twice a week during midlife can significantly cut a person's risk of dementia later, say researchers.

People in their late 40s and early 50s who do this could reduce their risk of dementia by about 50%, according to a study reported in Lancet Neurology.

Of course, it does nothing for those of us who are demented now.