US election's lesson for putting greyheads to work

There is this thing about age and retirement that has me constantly wondering as I watch the American presidential election campaign mercifully drawing to a close.
Around the world people get pensioned off in their early- to mid-Sixties, and some even in their Fifties. Here are two candidates competing for what is constantly referred to as the toughest job on Earth and both will be well into their Seventies during the course even of their first term.
I am not sure my choice of Donald Trump as an example of why there should be cause for reflection on this matter is of any help to my argument. I am not even convinced Hillary Clinton serves the purpose either. Nevertheless both were nominated by mighty party conventions, neither of which seemed to care in the least about their age.
In fact the Democrats even considered Bernie Sanders for the job, and had he made it to the White House he would have approached Eighty by the end of his first term. And keep in mind it was the younger set who were his most ardent supporters. They seemed to be the least worried of all about age as a factor.
If age then seems hardly to come into the reckoning in choosing a US president, why should it be such a strict disqualification for just about all other jobs around the world? In the process loads upon loads of experience and institutional knowledge goes to waste.
Why, in fact, should there be so much concern about aging populations in countries where people are having fewer children. The worry is about who is going to keep the economic wheels turning and pay taxes to keep such societies going. People are even encouraged to have more children, with no apparent concern about a global population that has shot up to over 7 billion in the space of just a few years and is putting unbearable pressure on natural resources?
If economic activity , contributions to the state coffers and less reliance on social services and state support are the concern, shouldn't the thinking rather go into ways of keeping people in gainful employment for longer? I think the advantages could be manifold.