The mentors frequently learn as much as the students. In the corporate world, formal mentoring programs make sure newer employees are learning all they need to be successful. But, just as importantly, these programs enable mentoring managers to sharpen their skills and become better leaders by interacting with their more junior colleagues.
( Source: HBR.org | Great Mentors Get Out of the Office )
I've had three mentors in my years. One was my professional mentor, another personal, and the most recent filled both roles. I didn't ever think I gave nearly as much as they gave me.
Now, as I'm leading teams and helping to shape the futures of young professionals (both inside and outside the corporate walls), I can look back and see that I did (and still do) give back to my mentors.
It's not a one-for-one swap, but I'm certainly keeping mentally fit in a mentor role.