We prefer to wait til an animal is 6 months for neutering because it allows the body's organ systems time to mature. The liver and kidneys have to metabolise the anaesthetic drugs, so we want them to have developed. It also means the animal is bigger - the smaller the size the higher the GA risk. That's why I hate anaesthetising rats/hamsters etc!
Some charities/shelters do animals younger, to increase their chances of adoption. There's quite a bit of debate amongst vets on this, but hey ask 3 vets any vet question and you'll get 3 different answers!
We (most of us) do like to spay bitches before their first season, as research shows that reduces their risk of mammary tumours later by 33%.
I'm so happy to hear about all the rescue animals FG members have
It's one of the things I really like about my British clients, how many of them have taken on someone else's "mistake". I haven't worked in Oz for a long time so it may be different, but people there still seemed to want the cute puppy or kitten, and they're still sold in pet shops!