And yet in the UK we had Boris Johnson and Jeremy Corbyn.
Sometimes people get in the frame because those who get them there think they can manipulate them more easily (Biden?);
Sometimes it's because they have popular support because of personal charisma (I'd say Trump and Johnson fit into this mould);
God knows how Corbyn got there but there was some evidence that a lot of Tory voters became Labour Party members so they could vote in the least credible general election candidate - also plausible with the Milliband brothers vote where the much less credible candidate Ed won over David.
However I don't think even we would have two candidates that show indications of dementia.
Interestingly the median age of leaders countries with free political systems tends to be lower than that of those who are not free and the median age overall is 62, so both Biden and Trump are way above that.
https://www.pewresearch.org/short-reads/2024/05/01/as-biden-and-trump-seek-reelection-who-are-the-oldest-and-youngest-current-world-leaders/
Personally I think 70 should be the cut off point. The risk of deteriorating physical and mental capacities is just too high above that.