Joe Biden turned 79 last week. He's beginning to look his age.

The president walks a little more stiffly now. His diction, once clear, sometimes sounds a little blurry. His syntax has always been ragged; that hasn't changed.

Like most of us, he gets tired at the end of a long day. At the U.N. climate summit in Scotland, he took his seat in the conference hall to hear the opening speeches and promptly appeared to doze off.

None of this appears to be clearly affecting his job. Last week he signed a $1 trillion infrastructure bill, a feat his predecessor was unable to perform at the comparatively young age of 70. He's in the middle of negotiations for the Senate version of his big social spending bill; if it passes, he'll deserve much of the credit.

There's no evidence that he's suffering from senile dementia, no matter how often ghoulish critics stitch together video clips of his gaffes. They seem to have forgotten that Biden has generated gaffes for half a century.

And yet the president's age still creates a political burden — one he doesn't need at a time when many voters have turned sour on him.

Voters, especially elderly ones who notice their own faculties eroding, worry that someone as old as Biden won't be able to do the job. He often faced that question when he began his third campaign for president in 2019. He often responded by challenging the questioners, including an 83-year-old Iowa farmer, to pushup contests.

Unlike a younger president, he has to demonstrate his continued competence every month. Unfair? Not really; the people he works for are entitled to ask.

And this is one place where Biden's White House hasn't served him especially well.

The president's schedule on his six-day trip across Europe was brutal, with long days, meetings with dozens of other leaders, plus phone calls to Washington to corral congressional votes for his spending bills.

All important priorities — but one result was a picture of an exhausted president that went viral.

In a different context, the incident might have been trivial. CNN chivalrously found old footage of President Ronald Reagan dozing off during a meeting with the pope, which was arguably worse.

For a president whose last electoral opponent dubbed him "sleepy Joe Biden," it wasn't smart to appear in public as a real-life sleepy Joe Biden. If he needed to sit in that conference hall, he should have had an aide by his side ready to kick him under the table.

Despite his age and his quirks, Biden is undeniably good at many kinds of public events. He can deliver set-piece speeches well, especially when he restrains his urge to wander from the text.

He's good at debates, town halls and news conferences, at least when he's well-rested and well-prepared. In 2020, he won his only face-to-face debate with Donald Trump hands down.

At a CNN town hall last month, he delivered a solid sales pitch for his spending bills, although his syntax was sometimes tangled. He mangled his descriptions of policies on Taiwan and National Guard troops as truck drivers in ports; that was vintage Biden.

But he's not as good off the cuff, or when he's challenged with a question he views as hostile. And, as we saw in Europe, he's not good when he's tired.

This is one instance where Biden and his staff can learn a lesson from a successful Republican predecessor.

Reagan faced the age question during his re-election campaign in 1984, when he was only 73. In his debate against Democrat Walter F. Mondale, he dispelled the issue with a quip: "I will not make age an issue in this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience."

More important, Reagan's wife, Nancy, and his closest aide, Michael Deaver, imposed iron discipline on his schedule to ensure that when he was onstage, he always looked his best.

We now know that Reagan was contending with early stages of Alzheimer's disease by the end of his presidency, but he found ways to compensate for his weaknesses. His actor's discipline and his willingness to rely on strong aides enabled him to finish two terms strongly and win credit for bringing the Cold War near an end.

The times are different now, and so is the president.

Biden, who has long been known for a stubborn streak, doesn't take stage directions as willingly as Reagan did.

"One thing I learned quickly," a former aide told me. "You don't tell Joe Biden what to say."

But if he wants to complete his ambitious agenda — FDR-size domestic programs, a revamped foreign policy, and maybe even a re-election campaign when he'll be almost 82 — it's a practice he might consider learning.

Doyle McManus is a columnist for the Los Angeles Times. Readers may send him e-mail at doyle.mcmanus@latimes.com.