Do polar bear cubs grow and become adults?
The wiki says their growth can be sped up, which would mean they do grow.
But expected they don't grow, like baby zombies for example.
Now this lead to another question, if you keep baby zombies or pigmen etc. alive long enough, do they grow into adults?
baby zombies of any kind do not have a growth factor and will remain babies forever.
Baby polar bears do grow and baby zombies will stay a baby forever