My mother died at age 77.  For the last 7 or so years of her life she became almost sedentary.  My Dad would say, "Harriet, let's go for a walk" and she would say no.  It disappointed him so, as he loved an evening walk, and he would have enjoyed the company.  He also hated to see her just sit in a chair.

I have often wondered if HER knees and hips hurt, as well as her back (we knew about that, as she had had spinal surgery in 1950) but she didn't say so.

Dad kept moving and lived to be 88, was fairly active right up until he died suddenly one night (dissected aorta.) 

When my first knee started to bother the surgeon I consulted said "You'll know when you are ready."  I had a scope with a minor cleanout, and kept going a couple more years. For me, the deciding factor was when I could no longer sleep at night due to the throbbing.  I still wasn't limping and I think that helped with my rehab, since I got my range of motion back quite quickly.

Now I have a lot of prosthetics--two knee replacements, one hip, and an aortic valve.  I am turning 75 next month and I am just about "bionic."  I also take walks--long ones--every day. 

Mother has been gone for 27 years and I miss her.  She never got to see her first great-grandchild, or our daughter's wedding.  I wish we had known then what we know now.