Dementia is a terrible thing, I've seen it first-hand more than once - and I'm currently in the middle of two cases. But then again I've also seen the opposite several times, people living to a very old age (95+, and, in a couple of cases, 100+) with all their faculties fully functioning and no health issues to complain about until it was suddenly and quietly over. It's not a given that we all (and our caretakers) will suffer in the end.