I have had both good and bad outcomes, and it really depends on the rat's condition. Even when the rat seems like a good risk, unforseen circumstances can still lead to an unfortunate outcome. I've mostly had good experiences having rats over two operated on, so I don't want to sound pessimistic, but I don't want to give false encouragement, either. I can say, however, that if the rat has no respiratory issues, is in otherwise good shape, and the vet is willing to operate, I'm generally inclined to try.
But you just can't get around the fact that, whether the patient is a rat or a human being, general anesthesia always bears some risk, at any age.