In reply to johnwarburton:
As far as the knee replacement and range of movement issue goes, it may well depend to some extent on your priorities with climbing and walking. Some people get the range to climb as before, some are compromised (proper rehab obviously essential). I am in a similar position to you (assuming you have osteoarthritis - imaging will confirm). My hillwalking is gradually becoming more affected and if I didn't climb I would probably be getting on with replacement, but I have been holding off for a couple of years because of the risk of messing up my climbing. At some point, possibly quite soon, the loss of hillwalking and, of course, mountain rock climbing will swing the balance the other way and make me take the plunge.
There is no perfect timing. Surgery has its risks and replacements don't last forever* but, on the other hand you don't want to end up after surgery wishing you had gone for it five years earlier and had been able to do a lot more in that time.
My advice would be to see a good, recommended, knee surgeon so that you know where you stand and definitely keep up the knee strengthening (physio will advise) - it will make your knees last longer and when you do have surgery rehab will be better with strong knees (don't end up inactive and weak before surgery by delaying too long).
*My surgeon said that 90% of replacements work well after 15 years and can last up to 30, so factor in your age!