When the second debate came around, Reagan put the aging question to rest with his now famous line: “I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent’s youth and inexperience.” A decade later, Reagan announced to the world that he had Alzheimer’s disease.

Joe Biden is 77, four years older than Reagan was during the 1984 campaign. If Biden is elected, he’ll be older on the day he takes office than Reagan was on the day he left office. So yes, his mental fitness is a legitimate issue.

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Any one of these gaffes in isolation would be nothing more than that. But taken together they form a pattern – and raise questions about whether Biden has experienced a cognitive decline. Biden’s defenders say this is unfair, and some have even suggested raising it is ageism. No, it’s not. His socialist rival, Sen. Bernie Sanders (I-Vt.) is 78 — almost a year older than Biden — yet no one is questioning his mental fitness. On Monday night, Sanders spent an hour at a Fox News town hall where he was challenged to defend his policies and answered in great detail and without any gaffes or senior moments. Could Biden do the same?

Many of President Trump’s critics have suggested that he suffers from cognitive impairment. Well, in 2018, Trump took a test designed to screen for dementia — the Montreal Cognitive Assessment — and the White House physician reported he received a perfect score. Will Biden submit to the same test?

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Sanders took a subtle dig at Biden during the town hall, pointing out that while he speaks for 45 minutes to an hour at his campaign events, Biden recently spoke for seven minutes. The two men will have their first mano-a-mano debate on Sunday. How will Biden perform over the course of a two-hour discussion? Perhaps he will put concerns about his mental fitness to rest. This much is certain: Democrats are about to pick a man they hope will be the first octogenarian president in American history. It is fair to ask whether voters are choosing a candidate who’s not up to the job.

Opinion Who could win the Democratic primary? Use the Post Opinions Simulator to pick a state and see what might happen in upcoming primaries and caucuses.