old head on young shoulders

an old head on young shoulders AND (have) a wise head on young shoulders (a child or young person who thinks and talks like an older person who has more experience of life) — умный на по летам; умный, мудрый для своего возраста; рассуждающий как большой /взрослый / умудренный опытом человек; ≈ умен не по годам

That's an old head on young shoulders!
Used to refer to a child or young person who thinks and expresses themselves like an older more experienced person.

Example: My little nephew said people who dislike other people don't like themselves very much. That's an old head on young shoulders. (CID)

Example: When she heard Emily warning her brother to stay out of trouble, her mother thought: "That's an old head on young shoulders."

(have) an old head on young shoulders AND (have) a wise head on young shoulders
(if you say someone has on old head on young shoulders you mean that they are wiser than you would expect for their age) — быть мудрым, умным не по летам; быть мудрым, умным для своих лет