This has been a useful thread, and that's a rare thing around here.
That study showing 44 and 60 being the big drop off ages is interesting. Just last week, I was talking to a running friend and commented that I saw a big performance decline at 45 (I'm late 40s now). He said he did too, and saw another one at 60 (he's early 60s now). That's perfectly consistent with the study.
I'm probably not old enough to meaningfully contribute to the conversation, but here's my experience:
I definitely get more sore and take longer to recover than I used to. I still run every day but take my easy days much, much slower. I do strength and stretching routine 3 times a week. I'm contemplating how much longer it'll be before I replace a run or two a week with some alternative. I'm resistant to the thought simply because I like running so much. I don't want to bike or swim, I want to run. Not because of any training effect, but just because I really like running every day. Eventually I anticipate that I'll have to anyway.
My race times (5k to marathon) were very consistent from 35 to 45, dropped sharply at 45, and then found a new plateau. That's fine, as I race for the love of doing it, not for an ego boost. I notice lots a fast-ish masters runners quit around my age when this slow down happens. I still love doing it regardless of the time on the clock.
I'm committed to getting a consistent 8 hours of sleep a night and rarely accept social invitations as a result (I'm an early morning runner).
Diet absolutely matters more than ever. If I eat junk, I feel like junk. Also, weight really matters. I feel an extra 5 pounds. Really feel them. I watch my calories hard.
I've never been a drinker and am thankful for that as the research seems to be saying more and more that the healthiest amount of alcohol is none
Genetics here is huge. Some guys seem to be able to keep sailing along (age adjusted of course) into their 60s and 70s, while others really hit a brick wall in terms of their ability to maintain volume and intensity. Some people can do everything "right" and still break down just because they didn't win the genetic lottery.