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. ;)
yes but worth the long term damage?
Yes, fixing your animal BEFORE it's first heat may reduce the chances of getting many different types of diseases. However, those diseases may be treatable in the short term. Fixing the animal later falls more into the "long term" care of an animal.
Fixing an animal later, allows them to experience some "sexual maturity". The horomone changes and bodily characteristics that come from this "change" are more important in my eyes. It allows the animal to mature properly and get rid of some of their "bad habits".
For example: What if you were "fixed" at age 13. A woman who never got a period or a boy who never got "stimulated" (Sorry folks :o ). Would your maturity level change who you are today? A woman never developing breasts and boy's voices NOT change?
That essentially is what is happening to our animals when we don't allow them to formally go through "Puberty". They may never get "breast cancer" but are they ever going to mature, calm, cool and collective? What would you rather have?
Ps. All my animals are fixed. One too early, the dogs just right, and the ferret already done. The one too early still has issues to this day 15 years later.