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.
Maybe a little consolation but still - you are doing an amazing and wonderful thing that you can be proud of for rest of your life. Extremely hard and taxing, no doubt there. Think about some nice moment from childhood when things are rough.
Respect to you and all others doing similar service.
From my pool of anecdotes I can only say that moving around - not necessarily doing sports or anything - affects things tremendously.
My maternal grandmother lived to 97, while my grandfather is still with us at 99. Interestingly, their next door neighbours are also alive and in their late 90s.
The one thing they had in common was living four floors up without an elevator. Curiously, in that block, people started dying starting from the lower floors.
Realistically I think they were born in an unique moment in history, which allowed them to have a childhood of running around barefoot, barely ever seeing a motor vehicle[0] and an adulthood with modern sanitation and post-war healthcare.
[0] Grandpa recalls how bewildered everyone was by the arrival of the nazis, with their tanks, motorcycles and, closer to the end of the war, bicycles(that part he found hilarious). They've never seen such a huge motorcade in their lives.