In the United States, most dogs are spayed between 5 and 8 months of age. Dogs should be spayed BEFORE they have their first heat. To try to control pet overpopulation, many animal shelters have started to spay/neuter all animals before they are adopted. This means they are spaying/neutering animals at a younger age, even 6-14 weeks of age. Many veterinarians in private practice have started early spaying/neutering as well.