Before I had any replacement surgery I would suggest getting a second opinion. There are many options out there that can be tried before partial replacements are done. I have no idea if you are a canidate for them but it would not hurt to explore all avenues before making such a decision.
Agreed.
In regards to people your age, it's definitely not common, but it has happened. I can't advise at all on the results, though. I imagine they'd be ok but of course the joint will wear out in time. There is optimism to be found in the high speed of progression in orthopaedics; what is not possible now may be in 5-10 years. Although a knee replacement would be a death sentence on, say, marathon running now, perhaps in 20 years it would not be.
In regards to when they will be done, they are a last resort and will depend on your pain and lifestyle. if you're in too much pain or cannot use your knee at all, regardless of the cause you'd be a candidate, I should think.