We prefer to neuter and spay dogs between the ages of 5-1/2 and 9-1/2 months, earlier for the small dogs and later for the large and giant breeds. Pets that have been spayed or neutered are less likely to get into trouble, because they are less likely to wander away. We know that spaying before the first heat will give the female dogs a 95% reduction in their chances of developing mammary tumors; they also cannot then develop an infection of the uterus (a pyometra) or uterine cancers. Neutered male dogs are at a much reduced risk of prostate disease, perianal tumors, and testicular cancer.
Cats are usually spayed or neutered between 5-1/2 and 6-1/2 months. Cats in heat or pregnant can also be spayed.