alt Hacker NewsThe cartilage in your knees is a living tissue. Like muscle, it responds and adapts to stimulus. If descending stairs gives you knee pain that isn't from some past injury being aggravated, then train them.
You get people who go skiing or running for the first time in a while and they complain "Oh, my knees hurt, I'm too old for this". Nope. You're just out of condition. Use it or lose it.
Nice theory, now go ahead and do it for some time (meaning few decades) and then lets talk. Or talk to knee surgeons.
Or no, just listen to few folks who have something to sell like books or training regimens or guiding, or simply won genetic lottery in that very specific part of their bodies, since such advice definitely applies to everybody, at any age, at any situation.