Join Date: May 2009
Location: Mid Northern TN
When working in shelter medicine, I am happy to spay and neuter as soon as the critters are big and old enough to hold up under anesthesia because the sooner they can be fixed, the sooner they can be adopted and go home. That is a special case. The longer they stay in a shelter, the more likely they are to get sick or develop behavioral issues, both of which are just about certain death sentences.
For animals in normal pet homes, I recommend they have their pet fixed after all their puppy (or kitten) vaccines are done, so typically 4-6 months. Surgery is a stress factor that can suppress the immune system a bit, so IMO, it's best to make sure they're protected from parvo, distemper, rabies and bordetella before doing it.
My only real exception to that general recommendation is in homes who do heavy physical activity or competition with their animals (Flyball, Agility, Herding, Dock Dogs, etc). There is some evidence that in some male dogs, early castration slightly increases the risk for orthopedic injury, so I recommend they wait until the dogs is fully mature, typically around 2 years old or so. Now, all that said, that's if you choose to alter your pet. Most of my clients with competition animals keep them intact because they may become valuable for breeding if they succeed. These folks are also typically exercising, managing, and training their animals enough that they're not running off and breeding willy-nilly.