My opinion - later in life
My opinion is that you should fix the animal AFTER the first heat or first litter. (First litter ONLY if intentional for breeding). The 6 months rule should correlate with the first heat anyways. For male dogs fixing should be done after the testicles have "dropped".
I say this from experience. My cat got "fixed" at around 3 months. It was the BIGGEST mistake! Her mentality is a little "immature" and never seemed to become a full adult. She is over 10 years old! Her physical status has been the worse effect. Everyone has commented that she is a "fat cat". However, she is NOT fat in any shape or form. Her "breasts" fell down and it is a lump of droopy skin! A fat cat's skin is taught well hers is extremely loose.
I've noticed other animals that have gotten fixed too early and they have "maturity" issues. That's just my opinion. My next time getting my animals fixed I will wait. I am just NOT comfortable with the trend of fixing animals early. I do see the point of it. There are too many unfixed animals without responsible owners. This provides a "protection" from encouraging more unwanted animals from neglectful owners.
Talk to a few vets and decide what is best for your animal. This is just MY Opinion. Please make your own.
Scooby, Shaggy the "Dogs", Ms. Thang the "Cat" and introducing Measley Weasle "The Ferret".