Counter-anecdote: my grandfather retired "early" (in today's view) at 55 and dementia wasn't a major problem until his 80s. He had several great decades of retirement. (Partly because he had several pensions and good social security.)

If anything, "standard retirement" of people only allowed to retire at 65 or 70 seems to intentionally pushing up against the limits of human stamina and decline hitting hard soon after retirement almost an intentional "feature" of the current standard because society can spend less on pensions and social security if we just require everyone to work until they break.

Anecdotally feels to me like more evidence we should roll back the decadal encroach of retirement age into way too late in life to enjoy it and return to better pensions and social services.

This is an example of sitting on the couch all day vs getting hit by a car for physical decline. Sure, there are extreme cases, but consistent long-term inactivity causes decline in every area of life. This is very well-known in circles researching stroke recovery - a lot of functionality can be restored, but only with significant active effort (both mentally and physically).

Terry Pratchett was about as mentally stimulated as a human being can be.

Perhaps that helped delay his case, perhaps it didn't, but it's very clear that stimulation is no guarantee.