You know, the publishing world is pretty small. It only grows when new writers and editors (frequently younger) enter the profession. I'm well into middle age, myself, but I don't have trouble working with people of all ages, and I hope they feel the same way about me.
Heather, while I understand (totally - I failed Prep my first time around) your disappointment, I have to agree with Del. You are in control of how you react to your current situation, but, just as you felt you didn't "click" with your mentor, you can't control how he felt about your writing or how well he felt he established a rapport with you.
As long as you did your very best, and I can tell you did, that's all you could have done.
Before you try to appeal the decision that was made, may I respectfully suggest that you think through all of your options? The same About.com staff will edit/mentor you should you decide to apply again. You may (as I did during my second Prep) have a different mentor, or you may not. Ask yourself whether you want to apply again some time down the road. Also, consider whether you would appeal a decision made by, say, a print mag editor who substantially changed a piece you submitted, or turned down a pitch you made.
While you may not agree with how About.com handles its application process, it works for them.
We're all here to help each other and be supportive. I certainly understand how you feel and agree that it's difficult to understand why things worked out the way they did. I imagine that there are many writers here besides Del and the two of us who have had to deal with the Prep experience, only to get the "Sorry, but..." email, and they also understand how much you've dedicated to the process and how frustrated you are.
I definitely don't want to tell you what to do. Your career is yours to manage. Still, important decisions require careful thought, in my opinion, and, based on what you've written here, I feel that the decision to appeal the editors' choice is an important one.
Whichever way you decide, I wish you the best, and I hope you'll consider applying for another Guide position some time in the future.