What was the question again !
The only time I have ever come across the suggestion that there is a 'sell-by' date after which you constitute a liability as a student/pilot, was in a book by Derek Piggott. I thought it was simplistic nonsense then, and I still do now.
The physiological requirements are simple enough to assess in terms of health, strength, articulation, motor skills and visual acuity; I think we all know that you don't have to be an Olympic athlete to be a pilot!
So that leaves the psychological issues of memory, situational interpretation and workload capacity. How you fare in both areas is down to a) the genetic lottery and b) lifestyle. By way of example, I have known people who are as sharp as a pin in their 90's and others in their 60's who don't know the day of the week.
In fact you could argue that the 'older student' population is self-selecting in any case. This is because all students need the motivation and learning ability to slog through the exams and the dedication to consolidate the muscle-memory for the practical aspect of physically flying the aircraft.
So let us please move away from the idea that ability is inversely proportional to chronological age. Time undoubtedly takes its toll, but as adults the vast majority will know when it is time to quit. Indeed, I have a lot of respect for those who have the maturity to do so.