Could it be that people who don't maintain an activity that requires a significant amount of mental activity begin to loose brain function. Just observing people who I know who have retired I have noticed a significant number of them have no significant interests. Once they retire they spend their days mostly sat watching TV, a very passive activity. It is these people who seem to loose their ability to cope with mental tasks, while others who have a hobby or some kind of interest that they are actively involved in seem to do much better.
I think there is some truth in use it or loose it, most striking example is my mother-in-law was very active, attending classes and involved in various activities. However after a stroke she was unable to attend these activities and over time it was obvious to see the decline in her mental capacity. It wasn't a sudden change caused by the damage caused by the stroke, but rather a decline over time. Of course some of it may just be age related but it does seem from observation that less she did the greater the decline in ability.
We don't know who it was that discovered water, but we're pretty sure that it wasn't a fish. -- Marshall McLuhan