Myself like many breeders and some vets. prefer to let large breed dogs mature before taking the sex hormones away (spaying or neutering). I have in my pet contract that males are not neutered before 18 months of age and that females are not to be spayed before 10 months of age. I would prefer that they go through one heat and aren't spayed before 15 months of age. The growth plates typically close at about 18 months of age in the golden retriever. The larger the breed the longer the growth plates take to close. Here are some articles on why we should let them mature (orthopedics/cancer) and also cover some behaviour myths.