re: Knee replacement. How did you know when it was time?

I know an ex offensive lineman that has had both? replaced. He says he should have done it earlier. He's in his 50s when he had it replaced. My grandmother couldn't handle three stairs in her house until she got a hip replaced in the 90s? First hip lasted ten years. Second one lasted over 15, and could probably be dug up and used in someone else for another 10 years.