Hector is
beautiful, JulieS!
A friend has two Bassets; Hector is almost the spitting image of the older one (in his younger days.) He had both castrated on veterinary advice at 6 months and both have infantile willies and have never really grown up. Neither had shown any testosterone-related behaviours and I never did know why the vet thought they should be castrated so young.
I was always told that 10 months was the earliest, 12-18 months better unless the dog is showing lots of testosterone-related behaviours. (In which case I guess it is already mature enough, huh.)
My first dog (that was mine as an adult, not the family pet) was a rescue Alsatian (as they were then) cross Lab (we think, being a rescue you can't be sure) and I got him at about a year old. He was always an absolute delight with all living creatures except with other male dogs, who he always wanted to subjugate. A male vet friend put me off having him castrated, telling me to try anti-testosterone injections to see what the impact would be, before taking the plunge. I did try the injections and the dog was practically comatose for a month - no interest in anything except lying in front of the fire, and food. So I didn't have him castrated, just developed tactics to handle his aggression with other male dogs. Eventually, partly because I worked away from home a lot and friends looked after the dog and my cats, and the friends were not so good at handling the aggression towards other male dogs, the problem became something I had to get fixed. So I got him castrated. I was told that, as he was a fully mature dog, it may take a long time to have an effect and that it may not have as full an effect as I would hope, because in dogs some of the testosterone is manufactured in the kidneys. The proportion and amount varies from dog to dog. Sure enough, there was little change for many months, but gradually he did become less aggressive with other male dogs and after about a year he was mostly quite manageable - although you did always have to be alert to doggy body language, and call him away if another male was getting a bit uppity with him.
Since then I always had all my male dogs castrated and bitches spayed when I got them (or at 14+ months for dogs, 7+ months for bitches if I got them younger) until I started having working collies. I was told that a castrated male would be less keen to work and so left my boys entire.
Ted, who became my farming partner's working dog on my previous farm and so stayed on that farm, was aggressive with other male dogs and that is how I came to have him - no-one else could manage him as he fought all the other working male dogs. He and Skip did fight if the opportunity arose, but Ted was so much better a fighter it rarely lasted long, although Skip did have a few sessions getting TLC and sympathy in front of the fire...
I have since had to have Skip castrated as his testicles had never descended and the vet was worried that that gave him an increased risk of cancer in them. So we had them out a year ago. He is definitely less energetic than he was before, and puts on weight very very easily, but his desire for and willingness to work is undiminished. He is actually a better sheepdog for it, as he has calmed down a little and listens better. (Of course I could have achieved the same effect by my being a better sheepdog handler! Or perhaps I
am becoming a better sheepdog handler, and it's nothing to do with his castration...

)
I have not had Dot spayed as I may yet want a litter from her - she is a tremendous worker and any 'spare' pups would be snapped up for working and/or trialling homes - but thanks for reminding me, Annie, that at some point I should think about getting her spayed for her own long-term health. (She's not 4 yet.)
So I think my take on it is that all pet dogs, unless specifically required for breeding, should be neutered at an appropriate age. I would not be frightened now to castrate a working dog or spay a young working bitch if there was a reason and I definitely did not want to breed from them.