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."
4.7 years. Also, no evidence that it reduces the risk of getting it, the benefit is purely delayed onset. Still, I'd take an extra 5 years of working brain, if only I could learn a second language (does Python count?).
