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.
Any other suggestions?