Juvenile Spay Neuter
Ask a veterinarian whether he/she spays or neuters dogs or cats under four or five months of age and the answer will almost always be "no." Then ask why, and usually the first answer you'll get is simply that "just don't do it." If you press, you'll find that most vets have no real prejudice against performing those procedures at an early age. Sometimes it's simply inertia. Sometimes they wait until the first series of shots is completed. Sometimes it's burdensome financially, on top of the other early expenses associated with animal care. Whatever the reasons, however, apparently there is no good reason not to spay/neuter early and there are some very good reasons to do so.
Those reasons have been succinctly presented in this ISAR Report written by Clayton MacKay, DVM, Ontario Veterinary College.