So, a couple of things. Play biting and teaching a puppy the limit of that behavior (essentially that it's not allowed under any circumstances) must be done during the first 6 months of a puppy's life. If you miss that window and the behavior persists, it's very difficult to extinguish (but not necessarily impossible). It's vital that a dog understands that a human hand can and will touch, probe and examine their mouths and the rest of their body at any time and that it's not a threat to them and that biting a hand even lightly is not allowed at all. Playing with a dog and allowing them to play bite is setting them up to fail IMO.