Dad is 85 and mom is 83. Both are still in very good shape cognitively speaking. My dad can still knock out the NY Times crossword puzzle in short order and has been volunteering to mentor young entrepreneurs at a local university. He made going to the gym and doing strength training part of his daily regimen when he retired in his mid 60s. He is no athlete, but gets around as well as anyone his age can. My mom reads a lot and takes lots of walks with her little fluff ball dog. She had to move into an assisted living home because my step father has Parkinson's and can't handle stairs well anymore and will need progressively more supportive care over the years. But she can still get around well and drive, cook, clean, etc.
At times, they do seem to get a bit confused and seem to take longer to understand some things. But I also have noticed that many times the lag is due to a generational gap and the fact that my parents have been retired and out of public live for 20+ years (and were never very with it to begin with). So, my father still doesn't get it that getting drip coffee has become an exception and espresso is now the norm at restaurants and coffee shops. He will ask for a coffee and then forget that an Americano is the espresso equivalent. He is then puzzled when he gets a latte and realizes that he got a milk based drink. Both parents have never been able to navigate the internet or smart phones well and constantly need help from their children or grandchildren.
So, I do not think that they have necessarily slowed down as much as I think that they reached a point where they just refused to keep up. I find myself in my 50s starting to do the same thing. Everyone in my office who is in their 20s to 40s is going nuts about ChatGPT and AI. I went on ChatGPT once and gave up after about 3 minutes. I have had no inclination to go near it since then. I am sure the 20something version of me would spend all weekend playing with ChatGPT.
If there were any lessons to be learned, it would be keeping up with physical activity, being open to new experiences and learning new things and making sure you are engaged with some sort of reading, writing, etc. as you age.