It's all about exercise:

Regular physical activity has been shown to reduce the risk of dementia, including Alzheimer’s disease, by as much as 50%. That’s according to more than a decade of research in the U.S. and abroad, says Dr. Erickson, who published a 2012 review of studies that involved thousands of participants in Archives of Medical Research.

How does exercise protect against dementia? The brain’s hippocampus and prefrontal cortex play dominant roles in memory formation and complex thinking, and their deterioration can be a predictor for Alzheimer’s disease. Amazingly, researchers are finding that these are the very areas responsive to physical exercise. In fact, higher fitness levels correlate with an increase in size in both the hippocampus and the prefrontal cortex. This means that exercise can help our brains continue to grow, and thus head off risk of cognitive decline.