an old head on young shoulders

Meaning

1. (have) an old head on young shoulders

   used to describe a young person who acts in a more sensible way than you would expect for a person of their age

From the same entry

• any old how
• any old
• as old as the hills
• for old times' sake
• the good old days
• of old
• old boy, chap, man
• old enough to be father
• old enough to know better
• the old story
• an old wives' tale
• one of the old school