I have an almost 14 week old lab pup who does the same thing. Part of it is the fact that they are just pups and put their mouths around anything. I am using a tactic that I've used on my other dogs before him, when he bites I tell him no bite and hit him softly under the mouth. He is bigger now and this seems to get the point across more. When he was smaller I used to gently squeeze his mouth together and tell him no bite. He doesn't bite me anymore, but he does attempt to bite other people who unsuspectingly begin to pet him. Other people have suggested rolling his lip under his teeth and squeezing until he cries a bit, I haven't tried this yet. I just tell folks to watch out because he's teething and if he does bite I show them how to correct him.
This is my 4th puppy and they all do it. Some stop earlier then others, but the important part is to be patient and consistent and firm in your corrections. Remember also there is a huge difference between vicious biting and playful puppy mouthing.