The best age to neuter a dog depends on several factors, including the breed, size, and individual health of the dog. Here are some general guidelines based on the search results:

• Small breed dogs (under 45 pounds projected adult body weight) can be neutered at six months of age or spayed prior to the first heat (five to six months) .
• Toy breed puppies or small breed puppies may be appropriate for neutering or spaying at six to nine months of age.
• Medium to large breed dogs should be neutered after growth stops, which usually is between 9 and 15 months of age. Waiting until the dog is at least over 6 months and likely even older for larger dogs is recommended.
• Large or giant breed dogs may need to wait until they are near or over 12-18 months of age. For male dogs, neutering after they reach full skeletal maturity is recommended, especially for large breed males. Waiting for skeletal maturity sets up new age recommendations for neutering: after 12 months of age for small breed dogs, after 18 months of age for medium to large breed dogs, and after 24 months of age for giant breed dogs.

It is important to note that the decision on when to neuter a dog should be tailored to each individual dog, especially if the dog is a large or giant breed. It is also important to consider the possible effects of neutering on the dogs health, such as orthopedic problems, cancers, and urinary incontinence. 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 health.