Nice one, Willie. This way of thinking will never put an end to your flying.
It is not one major issue that should stop you flying, if it comes to that you have been flying for too long. It is the slow way you body lets go over time, that is the issue.
There must be a line somewhere, before you kill somebody because of old age.
The medical checks are not designed to check for loss of memory, nodding off, slow reaction, poor eyesight in darkness, dementia, etc.
Eyes (in a well lit room), ears, ticker, blood and urine. That is pretty much it.