For many years I have had working dog people tell me that a dog should not be neutered or spayed until after they reach puberty. For a bitch that would be after the first heat. They claimed that the dog fully develops mentally after puberty and that early neutering and spaying retards this development.
In a recent study, scientists found that some breeds have a higher risk of developing certain cancers and joint disorders if they are neutered or spayed during their first year of life.
This study looked at 35 breeds over a ten-year period and analyzed thousands of dogs over fifteen years. What is interesting is that the age and sex of the dogs did factor into whether or not the dogs were afflicted with health issues.
The health issues under consideration included hip dysplasia, cranial cruciate ligament tears and elbow dysplasia, lymphoma; hemangiosarcoma, or cancer of the blood vessel walls; mast cell tumors; and osteosarcoma, or bone cancer. While the study showed that early neutering did not affect all of the dogs, there were breed related problems.
I personally feel that a dog needs the full range of hormones to fully develop mentally. But it can be difficult for the owners of female dogs to control a bitch in heat and prevent an unwanted litter. For the owners of male dogs there are methods to prevent the dog from siring an unwanted litter other than neutering. It is always wise to consult with your veterinarian and explore all of your options.
As an aside, I would like to see a similar study about the health and mental affects of early spay-neuter in cats.