Shifters are all special in their very own little way. Now, let it be known shifters don't just shift into any animal they want all willy nilly. You know their personality? Their animal form has to match their personality. So i don't want to see someone that's big and scary turn into a damn mouse, cause honestly that would just be ridiculous. Think of it as their spirit animal, but it doesn't really guide you anywhere, simply just shows your prominent personality traits. so for example, if someone is a dog person they aren't going to turn into a large cat. If they are quiet and shy, they might be something along the lines of a deer, maybe a shier breed of dog or cat. If their intelligent they could be anything from a pig(yes a pig, they are one of the smartest animals on the planet) to a dolphin even. But note, their animal form will dictate where they live! If they are a form of fish or say water mammal they will live near the water. A forest animal? Well duh near the forest. Those with forms of the nocturnal type will be night "owls". And they can only shift into that one single animal. There is no variations, no "able to change into any breed of dog" or "species" of wolf. One animal form and one only.
They are humans, they can get sick and will die from old age.
After a certain age, usually mid sixties it becomes impossible for them to shift, and no, shifting back and forth does not hurt unless they stay in the form to long. Yes, if you stay in the form or even out of the form for too long it will cause extreme pain when changing back and forth.