I think my Mom could still do it, but not my Dad. Same age, same health
regimen, same exercise, same diet, etc. So you never know, maybe you can, maybe you can't.
But if you don't, then you don't. That is easy and undeniable.
For me personally, I find the more I do, the more I can do.
Fitness happens with an active lifestyle. Cruising -- and by this I mean actually sailing around and across oceans, not just living aboard
-- is a very active lifestyle. The motion of the boat at sea works all your muscles, all the time. Your own activities add to this. So you can't help but get fit. When I spent last year cruising and hiking, I lost
all the weight I had gained over the previous few decades, and the degradation illnesses -- like back aches, sciatica, sore neck, joint pains, blood pressure, etc., -- all vanished. I mean vanished, no more.
But when I came back to LA, to the house, started driving a sports car again instead of being active, all that junk (pain and fat) came back. Very motivating to get the boat launched!