For many of a certain age, there comes a point when marking yet another birthday, taking note of yet another passage of the calendar, is not greeted with the same enthusiasm it once was. And that’s for people who are not even running for president.

For President Biden, who turns 81 on Monday, another birthday may bring more liability than revelry, offering one more reminder of his age to an already skeptical electorate. Unlike other presidents who have celebrated birthdays with lavish political events, Mr. Biden plans to observe his milestone privately with family in Nantucket later this week.

The best birthday gift the oldest president in American history could hope for would be a strategy for assuaging voters’ concerns, but that has been hard to come by. Mr. Biden and his team have taken the approach that his record of domestic legislation and international leadership should belie any worries about his capacity, even though polls have shown consistently that that line of argument has not been persuasive, at least not yet.

Some Democrats argue that Mr. Biden needs to do more to draw the contrast with his likeliest Republican challenger, former President Donald J. Trump, who at 77 has himself exhibited confusion in public lately. Mr. Trump has warned that the country is on the verge of World War II, mixed up the leaders of Hungary and Turkey, and boasted of beating former President Barack Obama in the polls when of course Mr. Obama is not running. If Mr. Trump wins next year and serves out four years, he would take over the status as the country’s oldest president.