I had a long discussion with this very topic with my vet after Pete, my rottweiler mix, was diagnosed with osteosarcoma (a horrible bone cancer that is more common in dogs that were neutered young.) I also looked at quite a few studies.
Bear and Pete were both neutered when I adopted them, they were 8-10 weeks old (it was done more like spaying is.) The rescue I got them from simply will not adopt out dogs that have not been desexed due to how unbelievably irresponsible people are in my area. I don't blame them but will never get a dog that was neutered that young again, because frankly (and my vet agreed) that is too young and does post a risk.
However there is no evidence that it stunted their growth or caused any of their health issues. Pete reached 130 lbs and got the characteristic barrel chest rotties are known for. Bear has a more athletic build, tall and narrow, and weighs 110 lbs.
She also feels a lot of the problems people try to associate with neutering before 12 months are most likely related to poor breeding practices, the severe amount of inbreeding that has been practiced for the sake of "purebred" dogs, rather than the age at which they were neutered. My vet and I both think the most likely cause of their health issues, besides that rottweilers and labs have both been subjected to a lot of "bad breeding," is that their father could also be their half-brother.
My other mutt Shadow, despite being neutered at 6 months, has had no severe issues that can be traced to any study claiming early neutering can be the cause. (At least I haven't seen a study tie FCE into neutering.) He is a sled dog/Belgian shepherd mix... we think. We do know he came from a sled dog kennel.
Sled dogs are a mix of any medium-to-large breed, mainly bred for working drive and stamina. Due to having no "pure bred" requirement to meet, no need for papers that say the dog is more valuable, they are a mix many breeds and are generally very healthy dogs.
One thing I could not tell from that study was, were they comparing the European dogs left intact to American dogs that had been neutered? The lines they are bred from are bound to have some major differences and be separated by generations. Just google "English Labradors" and "American Labradors" and you will see some pretty striking differences in build and facial structure. Have the American animals been subjected to more milling, inbreeding, and other irresponsible practices to meet the demand of the "pet industry" than dogs in other countries? It seems that way to me but I don't honestly know.
Now, forgive me if I'm incorrect, it's been a couple of years since the talk and my own research, but I found there was a higher rate of death in intact male dogs than neutered ones. Not only due to the increased risk of certain other cancers but by being victims of their own "wanderlust." Escaping and being hit by cars, attacked by other dogs, or in my area being shot due to ending up on the wrong person's property. They are also at increased risk of being abandoned or euthanized due to "aggression issues."
I probably should site these studies but I don't want to track them all down because of sheer laze. Of course like all studies they could very likely be biased or propaganda to persuade people to neuter, but I do try to rule that sort of thing out when I look into these things and try to find legitimate information.
My vet's stance on the issue was that 6 to 8 months is the common range for neutering. Neutering before that is "early" and poses a risk, and neutering later than that poses different risks and is largely unnecessary. She says she doesn't personally see a higher incidence of disease in neutered dogs.
Sorry for the long post.
Like I said, I looked into this a lot myself while trying to come to terms with Petie's cancer. I wanted to know a "
why." Why! Why was my Sweetie-Pete condemned to die of this at five years old?
There is no single answer.
