Some vets recommend castration at 6 months, in the same way that some also recommend spaying then, even if a bitch hasn't had her first season. But many vets advise to wait until later, until all the growth plates have closed and the dog/bitch is more mature.
There is evidence that early neutering before the growth plates have closed can lead to uneven growth of the dog/bitch, making it more prone to joint problems. Early neutering can also lead to the dog remaining in a puppy like state mentally for the rest of it's life. Hormones contribute towards mental as well as physical maturity.