The problem is that your life expectancy at 65, the year most leave work, is close to one year! That is if you work up to that time. Or, so I was told by an American actuarial scientist.
The age limit for the IMF MD role is interesting, but perhaps, it is better to think in terms of a medical that measures a candidate's 'real' age. It's true that some at 65 live on to be 94. My dad did! And, he was not senile until his last year.
Now, Charles De Gaulle lived on and on and on, but he was quite insane when he said 'Vive le Quebec libre'. Or was he?