The best age to neuter a dog depends on the breed and size of the dog. According to the American Animal Hospital Association (AAHA) Canine Life Stage Guidelines, small-breed dogs (under 45 pounds projected adult body weight) should be neutered at six months of age or spayed prior to the first heat (five to six months). Large-breed dogs (over 45 pounds projected adult body weight) should be neutered after growth stops, which usually is between 9 and 15 months of age. However, different breeds and different sized dogs mature at different ages, which means that early spay/neuter may not be bad for all dogs. Toy breed dogs mature sexually as early as six to nine months of age, whereas large and giant breeds may mature as late as 16-18 months of age. Recent studies suggest that the best age to spay or neuter should be tailored to each individual dog, especially if the dog is a large or giant breed. Therefore, it is recommended to consult with a veterinarian to determine the best age to neuter a dog based on its breed, size, and individual needs.