Spaying and neutering:

A very legitimate concern, pet overpopulation, has been the primary driving force behind 30 years of national and local spay/neuter campaigns. When it comes to deciding at what age a companion animal should be sterilized, the standard for most spay/neuter campaigns has been sooner rather than later... It’s unfortunately true that a growing body of research is pointing to early sterilization as the common denominator for development of several debilitating and life-threatening canine diseases. On one hand, we certainly want to know what’s causing our precious canine companions to develop disease. On the other hand, it’s troubling to learn a procedure we’ve historically viewed as life-saving and of value to the pet community as a whole, has likely played a role in harming the health of some of the very animals we set out to protect.
Think rationally. How would removing a child’s reproductive organs before puberty affect their growth, maturation, and development? Puberty and sexual maturation is imperative for bone, brain and organ development. The same is true for your dogs and cats... The best compromise, if any of these things is too much to deal with, would be to spay and neuter at a minimum of one year if not two years of age. Allow your pet to reach full maturation and reach adulthood before considering surgery.
Dogs that have been spayed or neutered at or before puberty can often be identified by their longer limbs, lighter bone structure, narrower chests and narrower skulls than intact dogs of the same breed. This differential growth frequently results in significant alterations in body proportions and particularly the lengths (and therefore weights) of certain bones relative to others... Early age gonadectomy was associated with an increased incidence of noise phobias and undesirable sexual behaviors, such as mounting. Significantly more behavioral problems in spayed and neutered bitches and dogs, with fearful behavior being most common in spayed bitches and aggression in neutered dogs.

But if an owner needs more certainty that a dog will not be bred, the answer is to perform a vasectomy. One possible disadvantage is that vasectomy does not prevent some unwanted behaviors associated with males such as marking and humping. On the other hand, females and neutered males frequently participate in these behaviors too. Training is the most effective solution to these behaviors.
The above data is just a small sample of the significant data that were determined in this study. By using large a sample of dogs than any used previously to examine behavior in dogs, we found significant correlations between neutering dogs and increases in aggression, fear and anxiety, and excitability, regardless of the age at which the dog was neutered. There were also significant correlations between neutering and decreases in trainability and responsiveness to cues. The other three behavioral categories examined (miscellaneous behavior problems, attachment and attention-seeking behavior, and separation-related behavior) showed some association with neutering, but these differed more substantially depending on the age at which the dog was neutered. The overall trend seen in all these behavioral data was that the earlier the dog was neutered, the more negative the effect on the behavior. A difference in bone length was found between neutered and intact dogs, suggesting that neutering has an effect on bone growth, which may be related to other orthopedic effects documented in the literature. Examination of changes in bone length of gonadectomized dogs is continuing.
The study, which examined the health records of 759 golden retrievers, found a surprising doubling of hip dysplasia among male dogs neutered before one year of age... the relationship between neutering and disease-risk remains a complex issue. For example, the increased incidence of joint diseases among early-neutered dogs is likely a combination of the effect of neutering on the young dog’s growth plates as well as the increase in weight on the joints that is commonly seen in neutered dogs... The study revealed that, for all five diseases analyzed, the disease rates were significantly higher in both males and females that were neutered either early or late compared with intact (non-neutered) dogs. Specifically, early neutering was associated with an increase in the occurrence of hip dysplasia, cranial cruciate ligament tear and lymphosarcoma in males and of cranial cruciate ligament tear in females. Late neutering was associated with the subsequent occurrence of mast cell tumors and hemangiosarcoma in females.
These studies have found correlations, not causal effects, and one can never be too careful about correlations... However One doesn’t need to be a veterinarian or physiologist to suspect that removing hormone-producing organs has a profound effect on a body’s physiology, beyond that of eliminating the potential of reproduction. Natural selection is a conservative process, and it is rare for any hormone to play only one role in the body. Androgens produced in the testes play a role in muscle and skeletal development. Estrogens, we’re told, affect the urinary tract, the heart and blood vessels, bones, breasts, skin, hair, mucous membranes, pelvic muscles, and the brain. Thus it is reasonable to wonder what the effects truly are of neutering animals at young age, sometimes as young as a few months of age.