My late husband was very mentally active- his consultant said his "starting IQ" (ie when he got dementia) was just about off the scale. He spoke several languages, wrote for a living for a while, taught for a while at college level, had several degrees, did art. and had an immense interest in absolutely everything! Fell down badly on the physical activity side though - wouldn't walk to save his life!
Personally, I think keeping the brain active may help - and William's consultant did say that his intellectual level and his educational level helped to stave off the EFFECTS of the dementia for probably years. The way he put it was, it's like driving along and finding the road in front of you has caved in so you have to detour. The more active your brain is, the more "neural pathways " it builds, so as the person keeps coming up against "road blocks" -damage from dementia - say they have lost the right words for something - they have a greater number of pathways to use to detour around the damage. So the dementia can go unnoticed for years. Eventually though, the damage becomes too great, there are no more detours to take, and then it looks like a very sudden deterioration. That's how William's illness went. I knew he had dementia. The doctors knew he had it. But he was still scoring very high on the tests until he was in the late middle stages of the disease.