Anecdote: My grandma retired in her 60's, she is now 102. She used to own a bar, in France (you can imagine the secondhand smoke, back in the 60-70's). She is mentally sharp, although the last couple of years, she is napping more. Vision (always had issue) and hearing (super annoying now) are declining, but memory is still here, and with hearing aid, the conversation is fine (if you do not mind repeating sometimes).
So got retired for about 4 decades now (she lost my grandpa at the same time, it is crazy she lived ~40 years with him, and ~40 years without him). Even though she was not "working" (i.e. for a salary) in the past 40 years, she always kept herself busy, taking care of her house until 2 years ago when she went to assisted living place (not ideal, but it was becoming too dangerous for her to stay by herself past 100 years old). She like to do stationary bike until her 90's.
Her daughter (now 82 yo) has some sort of Alzheimer, and does not have any short memory or facial recognition. Strangely enough she does have very old memories that looks current to her e.g. she warned us that we need to hide from the Germans... second world war trauma I guess, but interesting how it is still in her memory, when she cannot remember what she ate for lunch.
So, for a while like until 5 years ago, my grandma (97yo at the time) was taking care of my aunty (77yo at the time), and was schooling her to make here remember of things. That was weird to witness... My aunty retired in her 60's too, did not exercise more than normal, but was always active maintaining a large house after her 5 kids left home. She worked in a hair salon for about 50 years (started at 16yo). I always wondered if hair products (ammonia, spray, etc.) did not play a role in the current situation.
I have no idea how much exercise can slow down the decline, but there is more than just exercise. Clearly I would put my grandma as a "fighter" spirit person. She was born in an extremely poor family (alcohol issue, ending her as an orphan), in Brest, France which was where the Spanish flu started in Europe, in 1918 (she was one year old). She is a tough cookie, in a frail body. She is short and always been kinda skinny, probably the result of malnutrition in her early years. But yet, here she is a century later. To be fair, she is a bit tired to be around, and see everyone around her either die or getting some sort of Alzheimer. Now she is friend with her kids friends, since all of her generation friends are not there anymore. It is kinda cute to see her talking to my dad's childhood friends who are in their 70's or 80's now, recalling some mischiefs that happened 60 or 70 years ago.