I‘m with Barry here. My dog was neutered last year aged 4 but for medical reasons (he had an undescended testicle which posed a risk of turning cancerous). It has most definately spoiled his coat, he produces a lot more undercoat so I have to really keep on top of brushing and combing otherwise it gets very tangled. The hair on the bridge of his nose and the side of his face has completely changed, making him look much older. He‘s also become a lot more food obsessed, constantly looking to scavenge while we‘re walking which he didn‘t used to do.
Regarding his behaviour, - he still gets excited if he smells a bitch on heat, (although he doesn‘t pine anymore if there‘s one in the neighbourhood, and that IS an advantage). He seems to have lost a bit confidence around other entire males now and tends to growl and go on the offensive - however he has been attacked a couple of times, so its hard to say if its a result of the attacks or the castration.
I‘d never have been able to live with myself if his testicle had caused problems, so I followed the vets advice and went ahead but it does sadden me and I really wish I hadn‘t had to do it.
Adolescent dogs are no different to teenagers really, they try to flex their muscles and see how far they can go. They can try your patience but with consistency and positive training, they come out the other end and eventually calm down.
There certainly are valid reasons for castration but personally, I wouldn‘t consider behaviour issues as one, I‘d certainly try training first.