My vet prefers to do large/giant breeds at 1-2 years old. Spaying and neutering affects the growth of the dog. The earlier you do it, the longer the growth plates stay open. This doesn't let the dog grow properly. In a study done on large breed dogs, there was a higher risk of bone cancer developing on dogs spayed at six months than the chance of uterine/testicular etc cancer developing after two years. I would wait until atleast a year. Being around dogs that aren't spayed/neutered everyday, I have to say that these dogs have a much better body structure because they were allowed to grow properly. A great dane, being a giant breed with alot of bone/joint issues in general, should have that extra time to grow properly, in my opinion.