I think that decision, at least in the US, depends on your total physical condition and not just age. If you have health issues that make surgery and anesthesia dangerous, you will be refused because joint replacement is considered elective surgery. And they also decide based on whether you can fully participate in a rehab program that will make you stronger and or more mobile than before.

I think other countries sometimes have age limits on specific surgeries, or require you to pay out of pocket for them.

Other than age, have they told you why they say "No" to the surgery?