I think people forget when discussing subjects like this that road racing at the highest level is a team sport. There are successful riders -- stage and TT winners -- who continue riding in the peloton as domestiques, and some of them are very highly regarded because of their discipline, but even more so for their experience. They also are capable of putting aside their egos for the sake of the team. Stuart O'Grady would come to mind except he shat his nest with his recent doping confessions.
Yes, the riders in their 20s are going to be the stage winners, but they get there with the help of the rest of the team. And the good teams have a mix of age, experience and raw talent. I think we will find quite a few of the older riders extending their careers in coming years as support riders.