an old head on young shoulders
A young person who acts or speaks like an older person. Katie may only be 13, but she is so insightful. She's really an old head on young shoulders. Tommy seems like an old head on young shoulders since he's always repeating the outdated phrases he hears from his grandmother.
(have) an old head on young ˈshoulders
used to describe a young person who acts in a more sensible way than you would expect for a person of their age: He’s only seventeen, but he has an old head on young shoulders and remains calm under pressure.