There is no harm in neutering even a senior cat. It may keep him from getting testicular cancer, and it will certainly make his urine stink less, though it's doubtful he'll stop spraying if he's doing so already. Anesthesia has come a longgggg way over the years, and it's a very quick and safe procedure. A friend of my husband had a cat that she never bothered spaying, and the cat gave birth at 18 years old! The vet said not to bother spaying her at her advanced age, but she managed to birth one kitten anyway. Wonder she didn't die of pyometra, or mammary tumors etc. during all those heat cycles. The cat will be happier neutered!