[QUOTE=SonnysMom;8482189]
There is some research that suggests that neutering early on large dogs changes the bone growth of the long bones. This leads to the angles in the hips and elbowing being different between an unneutered dog and neutered dog. This in turn can contribute to elbow or hip dysplasia.[/QUOTE]
The mechanism of this is that the steroid hormones testosterone and progesterone cause the closure of the long bones in the legs.
Absent these hormones, the bones continue to grow, causing the early-neutered large/giant breed to grow taller, because the legs are longer, but often spindly in appearance. Early neutered dogs of these breeds are also more prone to orthopedic issues because of this abnormall growth. This is one of the major reasons breeders recommend neutering after the growth plates have closed.
Another reason is that many large/giant breeds are at increased risk for certain types of cancers- osteosarcoma, hemangiosarcoma, lymphoma, etc. Early neutering increases the likelihood that the dog will suffer from these types of cancer, as recent studies have shown.
So, because of the increased chances of orthopedic and cancer issues which has been shown by recent studies, most breeders prefer to delay neutering (if it is considered necessary) until after maturity, and in the case of bitches, to recommend ovary-sparing spays.
Certainly in toy/small breeds, or in situations where the dog is not going to be supervised/confined so that population control is an issue, that is a different situation.