One solution would be to downgrade after 65.
Once a pilot hits his 65 birthday, if he/she wants to continue to operate in a 135/91K environment than they can as a first officer. As mentioned by some, there are guys over 60 that have great skills and there are some who should hang it up, if they still want to fly, this will give them a chance, but they wont be in command.
I know its been posted about how aging affects the mind and I have taken enough human factors classes and read enough books to say this:
As someone gets older they do rely on their skills and past experiences to handle situations. There has been studies in which a young pilot fresh out of training is placed up against a "retirey" in challenging situations. It turns out that the young pilot actually takes a bit longer to react to a situation because they have to examine the situation, rely on what they have learned, make a decision, and then react. The older pilot through experience is able to quickly examine the situation determine an outcome based on experience and react.
That being said, the younger guy excels when it comes to them both learning new items and in stamina.
In our new world where technology and procedures change rapidly, I'm afraid that the "retireys" will be overtaken by the information flood. Sadly it's getting to the point where experience is getting replaced by technology. We don't have to "eye-ball" a thunderstorm due to weather technology, listen to a strange sounding engine because of advanced engine diagnostics, heck, our planes even help us with V1 cuts by pushing in on the correct rudder. The days of "seat of you pants flying" is over and technology has assisted in new information that a retirey could have only dreamed about.
All in all I think it would be fair to downgrade. Before the 65 age rule became what it is, if I have correct information, FedEx pilots who hit 60 were allowed to downgrade to Engineer if they wanted to stay in the cockpit.