Stosh just turned a year old and he still does the mouthing thing- I think he always will. He holds my wrist in his mouth and gives just enough pressure then releases before I say 'ouch'. I usually say 'gentle' and he knows what that means but still when he gets a little excited he nips. He only does it to me and my husband so I don't mind. Seems like a sign of affection. Now the nip at the face, that's another story! I definitely correct him for that.
Our recent rescue, Duke, is 5 years old and does this! When you just want to pet him, it's easy to get him to stop mouthing. But when he gets excited he nips at us sometimes. And for some reason he loves to nip at my husband's goatee and sideburns. LoL! My husband gets onto him and tells him no. He's done it a couple of times hard enough to sort of startle my husband and it has hurt just a little. But when my husband is petting him and letting him sniff and kiss his face, Duke always nips at him. We don't really know how to train this out of him either. I guess we'll try the "OW!" thing.
Kain is 9 mos old and he does this as well, always has. When I pet him, he'll turn his head just enough to place my wrist/arm in his mouth. He doesn't really bite down, he simply wants me in his mouth. I honestly think it's his way of telling me that he loves me back.