FWIW, I'm not concerned about age as much as I am wear and tear in college. Greene is indeed 24 but should have quite a few years left due to the less than average amount of usage he saw at Iowa.
Remember, fantasy is much more than just talent -- it's as much, if not more, about opportunity as well. Even if Thomas Jones comes back and plays for another year or two (which I really don't expect at this point), I expect there to be plenty of carries for Greene. And being that you have Jones already, that's yet another reason to select Greene here.
Two or three years from now... I think that McCoy will probably be the better, more productive back. He has higher upside. But there are also red flags for "Shady" (character issues, slight build) that don't guarantee success. If it was my squad I would take Greene.