Do you have a 'feeling' about how often you'd expect dementia to develop in most people over the age of 80 ? Internet says prevalence of moderate-severe dementia at age 82 is about 10%, about 20% for age 85-89. I think that's about my gut feeling area, too.
Without getting into technical definitions about difference between cognitive decline & dementia & 'memory problems'...
In my head, I think that it's normal, that if you die of a chronic disease after the age of 80, to get memory & cognition problems in the last 6 or so months of illness. I'm thinking illnesses like post-stroke, cancer, cardio issues, etc. Because it's normal to be mentally off-form when you feel quite physically ill.
My aunt who was sharp as a tack at age 80 apparently had chronic dementia for years before her death at 95. But her dad had bad memory problems from about age 75-80, so I thought aunt did pretty well, actually. I have other relatives who got to 80+ still very mentally competent, I help out a neighbour in his late 80s who is still a quick wit. I guess I shouldn't be surprised that he's in the 80%.