Kittens start getting their adult teeth between 4-6 months of age and in some rare cases they will hold on to a few of the baby teeth once the have grown in the adult teeth-this mostly happens with the k-9 teeth. I sounds like your kitten is testing his limits with the biting issue. Try and discourage this as much as possible as it can only lead to a monster later on! Good luck and keep lots of Band-Aids handy! I can say that too since I have a 3 1/2 wk. old kitten at home who is just now getting the 1st set of teeth!