My high school XC coach, for example retired 2 years after I graduated in his 60s (before quitting the team because he loved running too much to be a 250lb offensive lineman) and is still running 24 hour runs and multi-day 500k races well into his late 70s without any semblance of knee problem.
Although he played D2-level football in the 1960s, I wouldn't consider him a genetic aberration immune to commonplace orthopedic conditions like arthritis, bone bruises, or stress fractures.
After a few years of not running, I also let myself go and ended up getting passed in a 5k by an 80 year-old woman about a decade ago. I have seen something with my eyes that flies in the face of the "running is bad for your knees" mind virus, and it would be interesting to discuss if there were any longitudinal studies for people old enough to draw Social Security. I don't even think ultramarathon running is worse for your knees than being >200lbs, but I can't find the data that I think should be required before an orthopedist gives any advice one way or another.
I admittedly came into the assignment with an agenda because I thought the "most people should not run because knees" myth was created by people who don't run to deny personal responsibility for their health and discourage other people from believing they can improve their lives if they work for it.
Note: offensive linemen were a lot smaller prior to about 1985 than the 310lb behemoths out there today. If you like football, thank Lawrence Taylor for that evolution in the game.
There's absolutely some perfect middle ground of "just enough" running that will strengthen, but not deteriorate too quickly, your knees - but again where that point is will vary by individual. It also may not be something that can be determined except in hindsight, partly because medical professionals generally don't start monitoring cartilage until the person is reporting pain or mobility issues (or a known condition they're checking for symptoms of).
Point being that statistically there are useful trends in aggregate data that can be observed, but, paradoxically, those trends don't necessarily translate to good general medical guidance. One counterexample where those trends do translate would be something like that peanut allergy study from 2015 that was linked on HN recently about introducing allergens earlier and frequently to babies, resulting in fewer teen/adult allergies.
Okay, but I’m still noting that if you google this exact claim, numerous recent studies found that running is found to build cartilage, contrary to past assumptions