It was a condition of adoption that our Lurcher was castrated at 6 months, although by our local vet, rather than the rescue. The only exception was if the vet refused for veterinary/medical reasons.
My boy had a bad start in life and the vet considered him underdeveloped both physically and psychologically and said she wasn't happy to do it at that point. I was relieved as, personally, I didn't want him done until he was fully mature, partly due to bone growth, but also because he's a naturally anxious lad and I felt he needed the hormones in his system to help him cope with life and mature properly, but having signed the contract I had little recourse.
We contacted the rescue, who agreed to wait a few more months, as it was based on veterinary advice. We then left it until we got a phonecall from the rescue telling us we had to have it done 'now'. At this point he was 18 months old, 28 1/2 inches to the shoulder, had stopped growing and matured a lot. I would, in an ideal world, preferred to have waited until he was at least 3, as I know a lot of sighthound people say it can take that long for some male Lurchers to fully mature, but the vet said she thought there was no medical/veterinary reason to hold off any longer, so we were given no choice and had him done.
I was so upset and worried, particularly that he might become fear aggressive, as he was already scared of off-lead dogs, but in his case it didn't happen. (He's 2 1/2 now and has continued to steadily improve to the point of not really being bothered unless they rush up to him when he's on the lead.) Ideally, if down to just me, I would have waited at least another year, but in his case at least, it wasn't the disaster I was worried it might be.
Many, many years ago we had a large breed dog that our vet advised us to neuter at 6 months when he started being aggressive to other dogs. I didn't know any better back then and we didn't have the internet for research purposes, vet was also, supposedly, a behaviourist, so I agreed and went ahead. The consequences were dreadful, my boy became both dog and human fear aggressive and despite trying every behavioural avenue we could fine, including using Prozac and intensive positive training he never got any better. Of course I can't prove it was neutering him that caused the problem, he had had a bad start in life and was already showing signs of aggression, but, the timing was bang on for a massive deterioration in confidence and control and having researched since I am as sure as I can be that neutering was what pushed him over the edge.
I feel very strongly that 6 months is too young and to be honest, am always surprised that vets will agree to do it that young - on large breed dogs in particular. I will never take on a rescue that requires neutering at 6 months, by contract, again, as I feel strongly it has to be done on a case-by-case basis and should be what's right for that individual dog and that although rescues are obviously concerned about indiscriminate breeding etc, they have a responsibility towards the dogs they rehome, to do what is right for them, as individuals, rather than have blanket policies that can have serious consequences for some.