Quite simply shelters, rescues, HS's, etc. s/n the cats, dogs, puppies, and kittens before they adopt them because of the over burdened pet population. Presently there are approx 10 to 12 million cats/dogs/puppies/kittens euthanized every single year. And that is in Canada only. The States have the same numbers or worse. A lot of other countries aren't far behind.
The fine folks who work at these shelters, rescues, HS's, etc are just plain tired of having to go pick up an animal that has been dumped or not cared for properly or abused. They are also plain tired of having to choose which animal's number is up when it comes to being overcrowded and having to make room for the ones that keep coming in.
I know your question is when is the best time to get them fixed but that question is not an easy one to answer. Personally I have always had my pets fixed at/around six months except for my little one who could not be fixed due to seizure activity. All of them have lived to a ripe old age with only the normal issues of aging.
Unless you are showing your dog or you have an exemplary dog that will continue a purebred's line in this day and age with the overpopulation crisis I see no reason for not s/n your pet at six months. JMO of course.
Assumptions do nothing but make an ass out of u and me.
We can stick our heads in the sand for only so long before it starts choking us. Face it folks. The pet population is bad ALL OVER THE WORLD!