Part of the difference is compositional: Baby boomers now make up nearly 60 percent of registered voters over 65, up from 40 percent in 2016. As a result, Mrs. Clinton was probably fairly competitive among the group of voters who are 65 and over today, even though four years ago she lost those who were 65 and over. Compositional shifts might explain even more than this, if it turns out that the greater mortality rate of older men compared with women means that Mr. Trump’s older supporters have departed the electorate at a higher rate. But it seems unlikely that compositional change explains it all.

Another factor is a growing bias in polling: Young women respond at a low rate to telephone surveys on cellphones. Pollsters typically adjust their samples to ensure a representative number of women or younger voters over all, but only a few do so in a way that ensures they have the right number of younger or older women. As a result, most pollsters give more weight to women over all, yielding a sample with the right number of women in total but still too few younger ones and, as a consequence, too many older women.

Without any special adjustment, women might represent 60 percent or more of voters over age 65 in a typical high-quality national poll, compared with about 55 percent in reality. This probably expands Mr. Biden’s lead among seniors by about two percentage points — not enough to explain the whole shift but a component.

Together, the polling and compositional explanations might cover nearly half of the shift toward Mr. Biden.

What explains the rest? One possible explanation is Mr. Biden himself. Older voters appear to have a favorable view of him, and it’s not hard to imagine why they might see him in a more favorable light than Barack Obama, Hillary Clinton or even John Kerry.

He is the first white male Democratic nominee since 2004, and he is probably the first one since 2000 or perhaps earlier who won’t easily be dismissed as a liberal. He is not campaigning on the kind of culture war issues, like immigration, racial justice or gay marriage, that have tended to work to conservatives’ advantage with this group for decades. In other words, older voters may like him for many of the same reasons he disappoints young progressives.

Another possible explanation is Mr. Trump — and perhaps especially his coronavirus response. Older people are relatively vulnerable to the coronavirus and relatively insulated from the effects of an economic shutdown. The president’s drive to reopen the economy, and questions about his slow response, may resonate very differently for a retiree who backed Mr. Trump than for a young parent struggling to support children. A recent Morning Consult poll found that Mr. Trump’s approval rating on the coronavirus response was lower among seniors than among any other group.