If you're looking for optimal gains, polar bears are best around 60-65 skill. You can still gain beyond that, but you may find you gain better elsewhere.
I find all my tamers have had different points at which they felt ready for a change. Basically if your gains really slow down, look at the next animal on the training list. But continue taming the same animal if you're seeing good gains.
There's a full list of which pets to tame at specific skill points
here.
Wenchy