An extremely simple hypothesis: The world of today is more cognitively demanding. To take a very simple empirical example, 7% of 25 year olds had a college degree in 1960, 36% in 2020! You have to wonder what the neuroprotective effects are of being able to write an essay, analyze a novel, perform a chemical experiment, prove a theorem, whatever, over a lifetime. But it's well-established that the brain, like all other organs of the body, is "use it or lose it."