I had both of my knees replaced when I was in my mid-50's (I'm 61 now) and, as many others have remarked, it was one of the best decisions I ever made. On the flip-side, I know several people whose knee-replacement surgeries turned out to be disasters from which they never recovered. I'm lucky that both of my surgeries went without a hitch.
I found the rehab to be unexpectedly painful, but it's absolutely essential that you tough it out. For my first knee, I went to regular physical therapy sessions. For my second knee (replaced about six months after the first), I simply did my own rehab at a gym. Either way, the importance of diligent physical therapy cannot be overstated.
Finally, I agree that it's vital that you research ortho surgeons before choosing one to replace your knee(s). If you get a good one, you'll congratulate yourself for the rest of your life.