I think there are two things we need to do to prevent dementia, and both have already been touched on here; so I will just add my comments and observations on the subject.
I think that keeping our mind active is a very important thing. We need to keep thinking, keep planning, and then actualizing the plans. We should still have dreams and goals, and work towards them every day. Reading informative books and articles helps keep your mind active; so if you are interested in something--- study it !
The second thing that is important is to take care of our mind/body. Many foods that we eat are not healthy nourishment for our bodies. Feeding our body garbage is never going to produce a healthy body, and it tell your self that you don't care any more about your body than to feed it garbage.
Alzheimer's is known to be a type of diabetes that effects the mind; so avoiding diabetes is a good way to avoid dementia as well.
Our brain works and lives with cholesterol. We need it for our brain to function. Doug is right about eating eggs ! Our body can make cholesterol, as long as we give it the healthy food it needs to make it with. The problem with cholesterol is not having it in our bloodstream; it is when we eat foods that scrape our arteries, and the body covers over the artery with cholesterol to help it. If we don't eat the foods that cause the problem, we don't get cholesterol in our arteries.
Another thing that helps the brain function is coconut oil. They are actually using it as a treatment for Alzheimer's patients, and having excellent results. Coconut oil can pass the blood/brain barrier and be used by our brain as nourishment, so adding a spoonful to your food 1-3 times a day is an excellent preventative against dementia also.