A mentor can give you guidance through a tough spot in your career, allows you to see and think things outside the box, has contacts to reach out to, or even provides concrete next steps to consider when preparing for the turbulence.
People can benefit from mentor(s) in different parts of their life. It's never too late to have that someone special to give you advice.
However, young professionals, recent graduates or soon-to-be graduates, need some help more than anyone else. The best time to find that mentor for your first steps in your chosen field is often through alumni networks or internships.