have an old head on young shoulders

have, as a young person, wisdom, caution, tastes... that are usually associated with mature or older people

You must remember that a Capricorn child has an old head on young shoulders. Google

He has an old head on young shoulders and has a chance of making a powerful impression on the global game.  Google

He hits very hard and has an old head on young shoulders, picking his punch's well and knowing just when to unleash a final fight ending volley of shots. Google

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