@VanGoSunflowers it's hard to know. But for bigger dogs, prone to joint issues, I absolutely wouldn't castrate before their bones and joints are fully developed.
The thing is, vets used to say six months - and then they realised how much damage they were doing to dog health, so pushed it back by six months...but in larger breeds the situation in terms of their health and development doesn't change between 6 months and a year. A labrador (as most retrievers) is not physically or mentally mature until 24 months. You shouldn't really breed a dog before then - as they're not physically or mentally ready - so why would you (being frank) cut off the ability to produce hormones that bring them to that maturity?
I'd be interest to know what @SpanielsGalore vet says about it. As I said, my vets have posters on the walls saying they categorically will not neuter dogs when they're young and some pretty shocking numbers about the risks.
There was a study recently that found that 17% of Golden Retrievers developed joint issues and/or cancer when neutered before 18 months. It found that if female Goldens were neutered, their chances of suffering joints issues or developing cancer increased a ridiculous amount. They actually advised, as a result of the study, that female Golden Retrievers were never spayed and that males were castrated after 24 months - but ideally not at all. Obviously, that's Golden Retrievers, but I'd be interested to know if a similar study with a wide number of dogs was done for other breeds...