This isn't exactly a new and startling idea. It came up in the keynote speech at a dementia conference I recently attended. There are lots of suggestions about how to avoid dementia. Apparently the one suggestion that has solid evidence behind it is engage in physical activity.
No one promises that this will prevent dementia. After all, if it would, then no one who has been physically active all their lives would get dementia, and we all know that not to be the case. But exercise is shown to reduce the risk.
How much exercise? 150 minutes per week.
How strenuous does the exercise have to be? The equivalent of a brisk walk. This is a level where it is somewhat difficult to carry on a conversation but not impossible.
There were other suggestions given over the course of the day, but apparently exercise is the one with solid evidence backing it up. Other suggestions included:
The so-called "Mediterranean diet."
Keeping mentally active, especially by learning new things.
Since the exact cause of most kinds of dementia is not known, exact prevention strategies can't be known. But exercise is known to be helpful to health in general, so it seems a no-brainer to take the current evidence that it is good for the brain seriously.