People who would pass The Knowledge test might already develop dementia at a much lower rate.
Stuff like "I'm your grandson, dadsnames kid" didn't parse at all, they said not to bring up shit like that so we didn't.
We had to move him into a house because he kept using his POW training to break out of the dementia ward. Pickpocketing keycards and shit, he was a badass till the end.
He told me he was so jealous of all this new computer stuff I get to play with. I think about that a lot this year
Also, The Knowledge Test is, IIRC, passed only once and then you do not have to retake it for the rest of your career.
If a test passed in your 20s or 30s predicted accurately a much lower incidence of Alzheimer's in your 60s, that would be a remarkably strong early signal. Not impossible, but worth extra scrutiny. For the vast majority of old age conditions, we cannot predict them so long in advance.
If you get into a black cab in London and ask to be taken to any location, anywhere within 6 miles of Charing Cross, that driver will know immediately the right route to take without satnav. To get to that point they need to know 25,000 streets and many, many landmarks - statues, museums, galleries, restaurants, hotels, and so on.
I used to regularly drink with a cabbie in London, and he said that every time somebody got in the back his heart rate would rise, as he'd realise he's "on" and about to have to work out where to head from scratch. Within 30 seconds he'd have every step of the way worked out, down to which lanes at which intersections, no matter where in London he was or where he was going.
That's the thing that's likely going to reduce Alzheimer's.
It would also be interesting to look at sleep traces of cabbies.
In the same way that you have code smell.
Granted, some use GPS, but most only really use it for pickup