What age is best for preventive castration?

A number of studies have shown that castration is just as effective at reducing male associated behavior problems as it is at preventing them. This means that whether the pet is castrated post-pubertally (e.g. 1 year or older) or pre-pubertally (e.g. 2 months of age) the behavioral effects are likely to be the same. There is, however, anecdotal evidence that dogs that are sexually experienced are more likely to retain their sexual habits after castration, compared to those dogs that have had little or no sexual experience prior to castration. Recently it has been advocated that castration be performed at as young an age as is practical, to ensure that it is done before the pet has a chance to breed. This is most important in animal shelters since it allows them to ensure that every dog adopted has already been castrated. To date, studies have shown that castration is safe, and has no long term effects on health or behavior, regardless of the age that it is performed. Many shelters and some veterinary clinics begin neutering as young as 2 months of age. They report that the surgery is often shorter and that recovery is quicker and with less post-operative discomfort for these younger animals. Once dogs are adopted into their new homes, most veterinarians recommend waiting until all vaccinations are complete before admitting the pet into the hospital for surgery. However, if general anesthesia is needed prior to the vaccinations being completed for any other reason (e.g. suturing a cut, removing quills) this would be an excellent time to consider castration. In summary, there seems to be no behavioral or medical benefit to waiting until a dog is "mature" to perform a castration.

Taken from: http://www.pethealthcare.net/html/body_neutering.html