Just to throw in my two cents. I do believe in a more balanced decision process on the timing of spay/neuter versus a widespread view that all animals should be spayed/neutered at a particular age. The following is just my opinion based on articles such as the two links that Debbie posted, and is therefore open to (much) debate.
The intent of very early spay/neuter (under 6 month) is targeted toward reducing pet overpopulation, with not much regard for potential health implications which may occur years later in the animal's life. I believe early spay/neuter is a necessity for animal shelters and rescue organizations, and an option for reputable breeders to ensure that their pet-quality animals are not bred.
For most of us pet owners, spaying/neutering around 6 months is most likely still the best option just to make the pet easier to live with through adolescence. Teenage pups are easier to handle and train without the additional raging hormones.
Pet owners with an interest in entering sports or athletics,
those who will be absolutely responsible in not allowing a pet-quality animal to breed, may wish to wait until the animal is mature to spay/neuter as there is scientific research indicating fewer bone and other health problems in animals who are spayed/neutered at one year or later. These health differences may be negligible, but canine athletes will have more stress on joints/bones, so waiting may be a reasonable option.
Every pet should be considered individually as to the ideal timing for spay/neuter.