Surely fitness to fly and competency should be the two yardsticks by which flightcrew are measured?
I've flown with two gentleman who's capacity to operate deteriorated rapidly in the last few years leading up to 65. As long as the operator has robust enough proficiency check procedures in place, then this should not be an issue to retain some valuable experience. Raising the upper age limit is more suitable in scheduled commercial air transport than non-scheduled, because the simulator duopoly seem to have abrogated themselves of the responsibility of maintaining a sufficient standard across the industry in non-scheduled.