New Link to an older article Tables Determining The Best Age At Which To Spay Or Neuter | AKC Canine Health Foundation
This article is based on rather dated research approx. 10yrs old.
What is very interesting are the tables they provide at the bottom of the article.
Showing breed pre disposition to certain health conditions and by health conditions those that are increased from either the spay or neuter or those that are decreased from the spay or neuter.
I copied what this researcher overall "conclusions were" from the above link onto this post:
Conclusion So how do you reconcile all this information in helping make decisions for individual animals? Considerations must include evaluation of incidence of various disorders, breed predisposition, and health significance of the various disorders (Table 2 and Table 3). For female dogs, the high incidence and high percentage of malignancy of mammary neoplasia, and the significant effect of spaying on decreasing its incidence make ovariohysterectomy prior to the first heat the best recommendation for non-breeding animals. The demonstrated increased incidence of urinary incontinence in bitches spayed before 3 months of age and possible effect of CCL injury in bitches spayed before 6 months of age suggest that spaying bitches after 6 months of age but before their first heat is most beneficial.
For bitches of breeds predisposed by ovariohysterectomy to highly malignant tumors and for breeding animals,
spaying at a later age may be more beneficial.
For male dogs, castration decreases incidence of disorders with little health significance and may increase incidence of disorders of much greater health significance. For non-breeding animals, evaluation of breed and subsequent predispositions to disorders by gonadectomy should guide when and if castration is recommended. As dog breeders, you are a source of information for people seeking a dog for companionship, to show or work as a hobby, or to grow up with their children. As veterinarians, we are one of the guardians of safety and good health for all animals in our society. It behooves all of us to thoughtfully consider why we recommend spay or castration for dogs, to ensure we are not putting our own convenience above their good health. For every individual bitch or dog, careful consideration of their breed, age, lifestyle, and suitability as a breeding animal must be a part of the decision as to when or if they should undergo gonadectomy. - See more at:
Determining The Best Age At Which To Spay Or Neuter | AKC Canine Health Foundation
Please note what I have highlighted in red.
More research is very necessary specifically breed specific research. Some of course has been done but not nearly enough.