I think they DO grow out of it. Charlie was really bad when we first got him at 3 months, and now, at almost 5 he's learned the art of the "gentle bite".
Usually when "mouth rowdy" we can replace our hand with having him fetch a ball, or give him a chew toy.
Sometimes when he gets to rough, or it actually hurts though I hit him on the nose and make a loud noise like he really hurt me. (mostly because he did)
This has worked for us. The nose hitting is few are far between now, and only when i started doing that did he start to implement the "gentle bite".
They don't want to hurt us.. i think we just need to set very clear bounderies for them and that includes positive and negative reinforcement.