Being a mentor - at work, as a parent or to anyone that can use your expertise and input is greatly rewarding. Probably one of the best things about getting older and wiser is that some people are actually interested in learning from you!
I have had great mentors and remember all of them, even what I learned from whom. It is time to pay back and even a little forward.
When I retire, I do want to mentor more than I can now at work - thinking about some of the outdoor youth programs but also engineering / academics. You may make a big impact on a young person's life and not even know it ...
Edit: One thought to add - Personally, I like to have some younger friends. I make a concious effort to hang out with younger people. You can learn something from them too - plus, it keeps you young, especially if it involves exercise!
G