Sounds like, and this has been known for some time, social engagement and mental stimulation are critical for retaining good brain function. Generally these also help one stay happy, which in turn helps retain brain function, so it's a positive cycle.
This article seemed to take population data and extrapolate it in a way that may possibly be suspect. Two of the three studies were clearly correlative. The other did multiple tests on the same person, but did not appear to control for the quality of retirement. I would tend to suspect that it's probably more important to get the factors mentioned above than to receive money for it.