Is dementia preventable?

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!
Generally no. There are many different types of dementia but currently the only types that are preventable are illnesses due to head injuries/repeated concussions (ie by not getting hit in the head), avoiding toxic amounts of alcohol and the like.
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.
Some evidence. Some evidence exists that a healthy lifestyle can delay or prevent dementia: stay active, control blood pressure, control cholesterol, healthy diet, say positive. Avoid benzodiazepines.
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.