This has been studied before and the benefits of exercise are pretty clear in staving off any dementia. Presumably the increased blood flow to the brain, well really the whole body, is what makes the difference.
Piracetam, which has been discussed here before, is prescribed in Europe after strokes and to prevent dementia, by increasing blood flow. L-argenine also promotes blood flow and there are medical studies to back that up.
Anyone getting older would be wise to exercise daily and perhaps include one of the supplements (baring any blood thinning or other problems).