When I was 50 my knees hurt so much that when we went to the mall, I had to sit on the bench for a bit after walking in from the parking lot. I had a great Doctor who thought outside the box a bit and he told me I must build up the muscle around my knees. He suggested a bicycle because it would be a low-impact exercise so I went to Walmart and bought an 18 speed mountain bike reasoning that I was terribly out of shape and would need all the gears I could get. I was sure right about that, for a long time I could only ride on fairly level roads. After a while, I started challenging a hill every day. After months, I made it to the top without stopping.
Throughout my 50’s and 60’s I was really into mountain biking and my knees never hurt! Coincidentally, I wasn’t ever short of breath anymore and likely had the heart of a racehorse. In my early 70’s my balance deteriorated so badly that I had to retire the mountain bike and ride a stationary bike every night while watching the news. When the news is aggravating, I pedal faster.
I'm 80 now and still exercising and have only occasional knee pains.