I think the quote ties in with what else we know about Alzheimers: it tends to develop in those with smaller social circles, in those who experience less novelty and less healthy lifestyles. If the person is out in society learning a new language, meeting new people, learning a new instrument, exercising their minds' regenerative capacities, that's a great "counter" to aging.

If they're sitting at home, not driving anywhere, watching cable news all day, that's a recipe for rutted-grooves of thought and deterioration of the mind.