1. Tom has an old head on young shoulders.
2. He has an old head on young shoulders.
3. You can't put an old head on young shoulders.
4. Young Mark seems very knowledgeable and experienced for his age — definitely an old head on young shoulders.
5. That man would do this job very well, he clearly has an old head on young shoulders.
6. That boy would do the job well; he clearly has an old head on young shoulders.