This is an interesting topic for me…but isn’t actually new news. It’s been studied quite extensively for decades and I’ve posted quite a few links to Peter Attia podcasts and articles…which always contain a good mix of references to studies of large databases observed over decades. Not saying that 108 study participants over a couple of years doesn’t reveal something, but still…
For myself, I’d peg the surprisingly rapid decline to between 65 and 70 with (specifically) the degree of fast twitch muscle fiber loss after my 2 lapiplasties and subsequent lack of mobility. I assume that the realisation is a proxy for all the other biomarkers that would show the biological senescence 
I did not see this thread a few minutes ago.
It is a great topic.
Fits perfectly with my making my last skiing descent down the hourglass shaped cliffy gully down the center of Mammoth Mountain called “Hangman’s”. Threading the vertical narrow throat requires precise perfect execution or oooopsie bye bye.
https://www.mammothsnowman.com/2024/04/18/april-2024-photos-from-mammoth-mountain/.
I loved it since i was 12 years old, but at age 68 I went down and my bodymind screamed “no more safety margin left for this!
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When I turned 60, my wife informed me that I can’t go up on the roof anymore. But I REALLY need to get up there to try to find out where the rodents are getting in!
My Strava ap measures my pace and elevation change on various hikes throughout the Shenandoah National Park. It also has King of the Mountain segments in which it measures the time it takes a hiker to hike from a marker at the bottom of the mountain to the marker at the top. After being off the trail for about a year for family reasons, I started hiking in earnest about six weeks ago.
Last Sunday I hiked hard up one of those segments in 53 minutes. I was pleased until I saw that I had climbed that same hill in 46 minutes just three years ago-when I was 65. Going downhill fast in figurative terms.
Or perhaps you just peaked too soon!
My husband keeps reminding me that, if I hadn’t been so fit before I had my surgery, maybe I wouldn’t have noticed the decline. It’s a point of view, I guess 
I agree that it takes more than 6 weeks for a 68 year old to get back in shape after such a long layoff. But I also understand that my rate of decline is accelerating with each passing decade, and that my exercise and diet regimen does nothing more than slow down the rate of decline.
Oops. Time to solve Wordle and Connections because my brain also needs all the help it can get.
Another of Attia’s articles to bookmark for anyone interested…
It’s not too late to attempt a modest remedy to this age related decline even if you weren’t bothering at 40 or 60 (but with the same limitations as trying to address finances that’ve been similarly neglected)
I’m not sure if this is behind the “premium content” paywall (I’ve tried to circumvent it) but it’s a nice overview of the topic…and, coincidentally, the route I took to the realisation that failure to address my mildly elevated LDL-C had led to significant undetected ASCVD.