The answer, according to my research, is PROBABLY.
Several articles that I've read indicate that though there is no proven way to prevent dementia, there are ways to reduce your risk.
One of the biggies is diet. The number one recommendation is to go on the Mediterranian Diet, which includes a lot of fruits, vegetables, whole grains, and healthy fats. It does specifically mention fish, but we all know that there are other ways to get those fats, even omega-3, without eating a friendly fish.
Here are the basics of preventing dementia, according to a lot of scources, including cnn.com, webmd.com, dementaicarecentral.com, ehow.com, and more...
Diet: Go towards a more "Mediterranean Diet" which is rich in vegetables, fruits, whole grains, and healthy fats and eliminates processed foods.
Exercise: Exercise is good for the brain! Exercise at least three times every week.
Keep your heart healthy: eat healthy, exercise, don't smoke, maintain a healthy weight, good blood pressure
Mental exercise: Stimulate your mind with puzzles, challenging reading material, board games, playing a musical instrument, dancing
Protect your head: Prevent head injuries. Always wear a helmet when you ride a bike, always wear your seatbelt, and avoid sports that risk head injuries
Socialize: (copied directly from dementiacentral.com article) Older people who engage in regular social activities show less cognitive decline. One reason for this effect is that social activities promote new connections between brain cells.