Don't know. There is some evidence that doing different things other than your daily routine can keep our brains growing and making new connections.
Somewhat, maybe. Recent research strongly suggests that the same factors as in heart disease increase the risk of both ad and vascular dementia. These include high blood pressure, high blood cholesterol, excess weight and diabetes. So controlling these by diet, exercise, lifestyle and medication may defer onset and/or slow the course--maybe long enough!
Risk modification. Multiple studies on alzheimer's show that some dietary measures lower risk, and some vitamin supplements also may help, but all of this is complicated by a complex genetic picture which confuses. The rush-presb-st lukes studies indicate least incidence in socially active, well educated, and exercising clergy. If you have hypertension, homocysteine diabetes, high cholesterol, need to control risk.
In some cases. There is some evidence that multi-infarct dementia can be prevented through good blood pressure control, good cholesterol levels, and regular physical activity. Maybe not entirely preventable, but at least delayed onset.