We learn how to be an adult by being with adults.
Teenagers are on the brink of adulthood.
By being immersed in the adult world. Schools keep them in a holding pattern until they reach a certain arbitrary age when we think they might be ready to enter the adult world. With mentoring, a young person steps into that adult world on a regular basis, and interacts with a variety of adults. Mentoring moves a young person beyond the familiarity of the adults in his personal life and provides a broader expanse of role models. The expertise is out there in the world.