Omg! I was just asking myself this same question about a 3yob who I care for. He's been with me for over a year and he definitely has gotten better but still puts things in his mouth and licks things. I've just been repetitive with him and take the toys away that he is mouthing and throw them in the sick and say 'we don't put toys in our mouths. Yucky germs' and when I catch him licking things (like my window) I make him clean it off with a wipe (I go back after with cleaner). Like I said it has gotten better and now that I think about it, he used to always have his hands in his mouth too but I used to be on him all the time with that too and now I can't even remember the last time I had to ask him to stop.