I am often asked by owners whether or not they should have their bitch speyed or their dog castrated. There has been a growing trend for veterinary surgeons to recomend early and prepuberty neutering in both sexes. I am not for this and present some research below that gives my reasons for this and this should be discussed with your veterinary surgeon before any decisions are taken. My advice on neutering generally for each sex is given below.
Male dogs. On balance my advice is that unless there is a particular reason why you are considering castrating a male dog then it is generally not necessary but if it is required, then it should be done after the dog has matured e.g. around 18 months of age. Please bear in mind behaviour problems including aggression are very unlikely to be solved by castration alone and behavioural modification therapy needs to be undertaken together with castration so it is not a quick fix.
Bitches or female dogs, If a bitch is not going to to be bred from (and there is no necessity to breed as it is not automatically benificial for the bitches welfare), or after a bitche's breeding life has ended then speying will prevent pyometra or infected uterus which is a life threatening and serious condition. I am certainly for speying in these circumsances. However, the procedure should be carried out between 2 and 3 months after a season has finished and in the case of a maiden bitch (one that has not had any puppies) not until after the first season has taken place and using the same time scale between seasons as above.. Speying bitches with existing aggression problems can increase aggression and speying too soon after a season ends can cause irritability and aggression because of the timing of the hormones being produced at this time. However, with normal bitches speying will not produce aggression.
Follow the link buttons for the most recent research and papers I have found useful in this area