@jholland Now, that's a good question, but it doesn't always have an easy answer.
Here are factors the patient and surgeon need to answer when considering joint replacement:
First, can the person not only tolerate surgery, based on overall health, but the rehabilitation afterward as well.
Second, does the person have the ability to do the rehab? For example, someone who has lost so much muscle mass that he or she cannot walk and do post-surgical therapy, they may no longer be a candidate for a knee.
What is the condition of the underlying bone to which the prosthesis must be attached? If there is severe osteoporosis or necrosis (bone death) an implant may not be possible.
Does the person have an infection, like MRSA or other hard to treat bacteria, a suppressed immune system, or other condition that will impede healing?
Finally, does the person have the mental capacity to understand & the will to tolerate the pain of surgery and the work of rehab?

Ideally, one schedules a joint replacement as soon as the failing joint causes enough pain or disability to interfere with the activities of daily living and the ability to enjoy life. The longer one just "struggles along" the more the body compensates, altering posture, muscles, gait and stressing other body parts.

Have you been advised that it's "too late" for a specific joint replacement?
Sue