If you're asking about getting a knee replacement, that question is best answered by your Dr and an orthopedic specialist.

Good Morning.
As another commenter suggested, speak to your doctor about it.
Quality of life is important. I have seen 65 and over perceived as "old" but that is not old! Older, maybe. We have potential to live a long life - why not work on the quality part.

As you shared some information, I would suggest, again as someone else put up here, speak to your doctor. If you are not engaging in proper fitness training and nutrition for your age and lifestyle I would work on that first with the appropriate professional (I am an exercise physiologist and nutritionist - will defer to your healthcare provider first!).
Surgery can be a great thing but it is also very demanding. Learn you options and progress towards feeling better!