For me this was by far the most fun and adventures thing I have ever done. So fun in fact I'm planning my own Coast to Coast trip next summer.
That age group maybe a little bit to spread IMO. I'm not saying it can't work but at 13 and 14 most kids haven't fully developed yet and wouldn't really fit in with the older kids on the trip. It is possible though considering my best friend after the trip was twice my age.
I don't know if I could ever lead a cross country trip myself though. Last summer I led 4 week long trips and I was so ready to go home.. Kids just don't listen. I know I sure didn't when I did my trip but I guess that's all part of it.
Even so I think it will be a great experience for the kid and adult alike.