It's hard to really say from anecdotes. My uncle retired early and was sharp as a whip until 86 or so. Then decline hit him hard. There was no change in life circumstances, he just got old.

Also, I think you'll find that taking care of someone who can't take care of themselves is a lot of work. I had to do it for my mom for 6 months and its a ton of stuff. Talking to doctors. Arranging appointments. Etc.

"I think you'll find that taking care of someone" => I know you were writing this generically. And I'm just replying to this for the sake of all of us who do actually know what it's like taking care of someone.

But yeah. Holy shit this is hard. I've been doing this too. Had to move my mom and dad to a place a block from me when my mom was going through her final few months with Alzheimers. That was so hard. So gross. And then now with this descent of my dads. You are catching me fresh from yet another aorta aneurism surgery of his last week. This is bananas. Just endless worry, driving, appointments, cleaning, pills, macgyvering the endless broken down things in his life: the tv, the remote, the blood pressure monitor.

OMG. I see you. I feel you. :) This is a rough part of life y'all.