Sudden Incapacitation
The stated reason in the preamble to the original rule on Age 60 was to avoid sudden incapacitation. What is the most common form of incapacitation you might ask. According to an FAA report it is digestive problems.
Take a look at the Qantas incapacitation a couple of years ago. The crew ent out for sushi the night before and the Captain became incapacitated a couple of hours into the flight. The F/O continued on for many hours, overflying suitable airports and landed (feeling queasy).
Let's solve real problems.
Goldfish