I have an 11 month old German Shorthair Pointer and have yet to fix him. I was told by a dog trainer I know that he recommends waiting until your dog is mentally mature. With a GSP that is between 18-24 months. If we think about a castrated buck here, they typically dont lose velvet or shed antlers and dont rut the way intact males do. The hormones in the testes/ovaries dictate a lot of the physical and mental phases animals go through as they mature. Im not sure if theres any research to back up his next claim, but he also claims that fixing a male dog before he mentally matures essentially renders you with a physically full grown dog with the mind of a juvenile which may cause behavioral problems that are harder to correct later on. (however, my last dog was a chocolate lab I fixed at 7 months and he was a great dog, although I will admit he had the temperment and behaviors of a puppy most of his 13 yr life).
I'm not sure if all this is true or not, but regardless, I have a couple people who want to breed their dogs with mine before I snip him so he will most likely be around 18 months before I have the procedure done.