miệng còn hôi sữa, quá non nớt = to be young and without experience = lacking experience; immature
He's a nice young fellow but a bit wet behind the ears.
If you say that someone is still wet behind the ears, you mean that they have only recently arrived in a new place or job, and are therefore still not experienced.
You know I have started my position for a few weeks so then I have still been wet behind the ears.