I don't think there is a cutoff age, necessarily. My oldest cat is 10 and in perfect condition. I would have very little reservations of putting him under, if necessary. If he appears to be the same when he 15 I would probably say the same thing. Of course there are ALWAYS risks when anasthesia/surgery is involved but if the surgery would better the quality of the cats remaining life, I would go for it as long as they are physically healthy enough