mother's milk hardly dries on lips 乳臭未干，形容人幼稚

【例句】
Your mother's milk has hardly dried on your lips and you want to go into the army!
你还是个乳臭未干的小子呢，竟然想着要去参军！

His mother's milk has hardly dried on his lips, and now is not the time to go far.
他还乳臭未干，现在还不是出远门的时候。