Biden is the oldest serving president by three years - Ronald Reagan was 77 at the end of his second term. But Biden is only half-way through his first term, and at the moment seems intent on running again. He will be 82 on Inauguration Day in 2025, and would be 86 at the end of his second term. Donald Trump, who is definitely running again, turned 76 in June. He will be 78 on Inauguration Day in 2025, and would be 82 at the end of a second term. So if 2024 is indeed a Biden-Trump rematch, voters will have no choice but to elect America's first octogenarian president.
Is this a bad thing? Konrad Adenauer served as (West) German Chancellor until he was 87, when he (reluctantly) retired while still in full command of his faculties. There is no necessary connection between ageing and cognitive decline (what used to be called "senility"). But the fact is that most men* over 70 experience some degree of cognitive decline, and both the incidence and degree of decline increase in men over 80. In a minority, this leads to dementia, but for most it is a manageable condition. But the principal reason it is manageable is that most men over 70, and nearly all men over 80, have ceased work, and so the demands on their cognitive faculties are much less than they were earlier in their lives.
There is therefore a definite risk in having a man approaching or over 80 holding the most powerful and demanding job in the world, President of the United States. There is a reason why airline pilots and military officers, among others, have mandatory retirement ages. Although some men are perfectly capable of doing these jobs at 86, many are not, and the risk is thought to be too great. But no such restriction applies to the job of Commander-in-Chief of the most powerful military and the largest nuclear arsenal in the world.
If Biden wins a second term, I would be reasonably confident that he, and if not he then his wife, his doctors and his staff, will be aware of this risk and will monitor his health and his cognitive abilities closely. I'm also confident that they will be willing and able to tell him if they think it is time to quit, and that he will follow their advice.
I have no such confidence about Trump. He has a total lack of self-awareness, he thinks he is a "stable genius", he has consistently lied about his health (as about everything), and he has surrounded himself with toadies who will be too afraid of him to tell him the truth. Many competent observers believe that Trump has been mentally ill for years. This is just one of many reasons why re-electing Trump would a very dangerous disaster for the US and the world.
It is of course by no means certain yet that either Biden or Trump will be their party's nominee for President in 2024. Biden thinks, probably rightly, that he is the best candidate to defeat Trump. So long as Trump is the likely Republican nominee, Biden will remain determined to run again. With two elderly candidates, age will not be a campaign issue. But if Trump either drops out or is defeated in the Republican primaries (which looks increasingly possible), and the Republicans nominate a much younger candidate (Ron DeSantis will be 46 in 2024), then Biden's age certainly will be an issue, and his party will probably pressure him into retirement.
*I say "men" because the incidence of cognitive decline in men is higher than in women. This is apparently because cognitive decline is related to a falling level of testosterone, and men lose testosterone earlier and faster than women.
Pope Francis is 85, turning 86 next month and he's still pope, albeit his health issues have limited him to the wheelchair.
