good head on one's shoulders, to have a

have a good head on one's shoulders

Fig. to have common sense; to be sensible and intelligent. Mary doesn't do well in school, but she's got a good head on her shoulders. John has a good head on his shoulders and can be depended on to give good advice.
See also: good, have, head, on, shoulder
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

good head on one's shoulders, to have a

To be intelligent or shrewd. This term began life in the sixteenth century as having an old head on young shoulders, meaning a young person who has an old person’s wisdom. Sometime in the course of time “old” was clarified into “good,” although in the nineteenth century it was also put as simply having a head on one’s shoulders.
See also: good, have, head, on
The Dictionary of Clichés by Christine Ammer
See also:
  • do one's heart good, to
  • do no good
  • for good measure
  • be as good as new
  • do (one) a/the world of good
  • as good as it gets
  • (one) (has) never had it so good
  • (as) good as new
  • as good as new
  • expecations