Currently, DC-3s can continue to fly as long as they are certified airworthy, and age doesn't enter into it. Is there a reason why this should not also be the case for pilots?
There is an excellent reason why your comparison of the DC-3 to humans is completely flawed. A machine wears, it doesn't age. Aging and wearing are two completely different animals. A machine does not process brain. A machine doesn't know if it is operating during daylight or dark and does not cope with circadian rhythms.
If you take your comparison of a DC-3 it is extremely flawed. There are not too many DC-3 flying that contain 100% of their original parts. In fact if you compare the engine to the heart of the airplane I suspect there is not ONE DC-3 flying on its original engines. If it is then it would have had them overhauled many many time. Do that to a person and you lose your medical.
For arguments sake, say you took a new DC-3 off the assembly line brand new in 1935. You put that airplane in a hangar and don't fly it, but you maintain it in new condition. No corrosion, keep the engines pickled and it in like new condition until today's date. That airplane is 85 years old. In this case the airplane would meet those performance specs from flight manual spot on. The airplane/machine doesn't know age so it has the same performance at 0 years as it does at 85 if maintained in like new condition.
Now we take an 85 year old man. He has maintained good health and you ask him to run a 40 yard dash in the same amount of time as he did in his teens (remember machines don't age so to compare we will take the theorized optimum performance age of adults). He is not going to be able to run as fast in his mid 80's as he did in his 20's. Heck most of the gummers I fly with can't find their car in the employee lot after a 6 day trip.
A human is a continuously operating entity. A machine like an airplane is started and stopped. Machines can be completely shutdown and rebuilt with NEW parts from the factory. Humans not so much and the aging process for humans and machines is very different.
65 in the US was only about money. The senator (Ted Stevens) that backed the legislation to push age 65 through on a rider to a bill was forced out of office due to a government indictment for misdeeds and the coup de' grace is that he was killed in an airplane crash last month by an over 60 pilot. Sometimes there is justice.