Especially with large breeds, neutering early, before the growth plates close, can cause joint issues. Neutering slows down the closure of growth plates and depending on the age you neuter, some growth plates will already be close, as is natural, but others will stay open abnormally long, causing those bones to grow to a longer length than they were supposed to, which puts stress on the joints.
Citing a decrease in testicular cancer from neutering before a certain age strikes me as funny, since neutering at any age, as long as it is before the dog gets testicular cancer (which isn’t all that common anyway), completely removes the risk of testicular cancer, since the testicles are completely removed.
I have read a lot of research recently reporting that neutering may actually cause more behavior problems than it prevents, other than the desire to breed and those related behaviors. Neutered males are often cited as the aggressors in dog fights. Neutering seems to remove some caution as well as removing body parts.
If you are prepared to be a responsible owner, then neutering may be unnecessary. However, if you doubt your ability to keep your dog at home, under control, then neutering is better done early, before the dog gets to breeding age, regardless of other issues.
