The age restrictions used to be associated with the statistical frequency of serious, sudden, medical conditions, especially heart attacks.
A couple of things have happened since they were set up.
1. We know a lot more about heart attacks.
2. The incidence of heart attacks from 60 + yrs old in the past, now takes place at 70 + yrs old.
ICAO, EASA, FAA and CAA would be unable to justify the 65 rule in Euro/other Courts given current medical incident "averages"
However, I'd have thought that the variation between individuals is so high at 65+ that the suggestion above for tougher, individually focussed medicals would meet the needs of safety.
Balls out
The 60+ restriction was not re-instated due to the ICI incident. A number of Directors of a Division of ICI were killed due to a medical condition of a single pilot flight. The ICI gentlemen had rushed around looking for a way to get home when the commercial flights wouldn't do it. The big issue from that incident was single pilot flying for commercial purposes. (Was it also a single engine flight?) So that bit of history belongs more in the "single pilot for commercial flights" thread.