Biting in kittens is natural. They start biting each other around 5 weeks of age, and from then until 12 weeks the rough housing with each other is wild with lots of pouncing, chasing and biting, but this is a crucial time in a kitten's upbring when it learns to control its bite. If a kitten is taken away from it's litter mates too early, say at 6-7 weeks, it often hasn't learned to control it's bite. I was involved in the cat fancy for many years and never sold kittens before 12 weeks of age. None of my kittens were biters. If they went after my fingers, which all kittens do, I would say "No biting!" sternly and give a
light tap on the nose. That's what a mother cat would do and if kitty continued to bite would get a harder tap and maybe with claws out, or a bite to the neck. This is a natural thing with all kittens, tame and otherwise. It's practise for subduing and killing prey, but kittens can learn not to bite people. Too often
over-stimulation is a cause for biting, especially rubbing a cat's tummy. Some cats cannot tolerate it, or only for a short time, others just love it. All cats are different.
