That article was based on only a few studies done well over 10 years ago, which doesn't really seem very convincing to me.

I haven't had any problems with my dogs "sagging" and they were both spayed at 3 months of age. However, this is merely anecdotal evidence- but in my opinion sagging has to do with the competency of the vet- my pets' vet is a very good surgeon and the incision was very small and healed quite quickly. In fact, the younger the patient, the faster it typically takes for them to heal. I think pets should be spayed/neutered before or around 6 months.