After a search of the web, it's hard to say that I found anything conclusive written by an authority (an actual vet or vet tech would have done fine, but none were quoted in the articles that I researched.) A number of people referenced their veterinary professionals as their sources.
What I found was a few common themes:
- Neutering cuts down on aggressiveness when females are in heat - I'd expect that this would be the same as for any dog neutered when they were younger. Since most people don't live in a hermetically sealed bubble, a male is going to find some way to get to a female in heat whatever it takes. If you walk your dog, it's going to be that much tougher around others when this is going on. This leads to...
- Males running away - Over, under, and through fences. I've seen all of the above. Pretty universally I've seen through my reading that...
- Breeders are of the opinion that it's OK to not neuter your dog - The exceptions being for some that if a dog is ill, they're OK neutering it. That said with so many dogs in shelters, having unneutered or spayed pets around after their breeding years are behind them is a tough sell for me as a pet lover. Take the pressure off them and give them a few years where they're not worrying about how to get their next piece of tail.
- Dog-on-dog Aggressiveness - Like a schoolyard bully, even the smallest dog is going to want to stake his claim on the local female population when another male is around. Unfortunately I've experienced this with a dog that's neutered, that has made a couple of unaltered smaller dogs feel pretty defensive, despite being very happy to meet them. (A snarling beagle has nothing to prove to an 80lb. shepherd-mix, so me and the big dog just stay clear of situations like this.)
- Neutering reduces health concerns - There were a few folks out there who didn't agree with this, all were breeders, but the overwhelming majority spoke to a reduced chance of an enlarged prostrate as the big health concern.
- Finally... Neutering later in life doesn't significantly change the dog's personality - During the time when females are in heat: Yes, I've listed the changes above. Some males who have been neutered don't have "the drive" anymore. This could translate to being a little slower, but since that's also a symptom of getting older, it's hard to say for sure that these are related.
A dog neutered later in life will still have the good (and bad) habits that it learned earlier in life, including being aggressive to other dogs. Keep in mind that while it's not a panacea, neutering is not harmful to the dog and will take some of that extra pressure off him as he heads into his golden years.