I have bad knees from playing lots of competition squash in my earlier years. A surgeon some years ago suggested-put off surgery for as long as possible until the balance between function and pain becomes unbearable. Good advice I accepted and now very conscious of not overdoing anything that overloads the knees eg strenuous exercise; walking on sand. On the other hand a colleague in his 50s had both knees replaced last year and could not be happier with results. The calibre of the surgeon and their advice is critical.