The decision on Thursday not to file criminal charges against President Biden for mishandling classified documents should have been an unequivocal legal exoneration.

Instead, it was a political nightmare.

The investigation into Mr. Biden’s handling of the documents after being vice president called him a “well-meaning, elderly man with a poor memory” and described interviews in which he could not recall when he served as vice president, what year his son died or whom he agreed with during policy debates.

The memory of the then-80-year-old president was so hazy during five hours of interviews with the F.B.I. investigators over two days, according to the report by Robert K. Hur, the special counsel, that it would be difficult to convince jurors that Mr. Biden knew his handling of the documents was wrong. Mr. Hur predicted in the report that if the president were charged, his lawyers “would emphasize these limitations in his recall.”

In part because of Mr. Biden’s memory, Mr. Hur declined to recommend charging the president for what the report described as willful retention of nearly seven million pages of national security secrets, including some documents shared by the president that implicated “sensitive intelligence sources and methods.”

“It would be difficult to convince a jury that they should convict him — by then a former president well into his 80s — of a serious felony that requires a mental state of willfulness,” Mr. Hur wrote.

In his own statement, Mr. Biden appeared to suggest a reason for why he was distracted.

“I was so determined to give the special counsel what they needed that I went forward with five hours of in-person interviews over two days on Oct. 8 and 9 of last year, even though Israel had just been attacked on Oct. 7 and I was in the middle of handling an international crisis,” he wrote. “I just believed that’s what I owed the American people.”

The president’s lawyers, Bob Bauer and Richard Sauber, took exception in a Feb. 5 letter with Mr. Hur’s description of the president’s memory.

“It is hardly fair to concede that the president would be asked about events years in the past, press him to give his ‘best’ recollections and then fault him for his limited memory,” the lawyers wrote. “The president’s inability to recall dates or details of events that happened years ago is neither surprising nor unusual.”

Concerns about Mr. Biden’s age have been a recurring theme of his presidency over the past three years. Fueled in part by video of the president appearing weak or stumbling in public, many voters have expressed concern about his mental and physical fitness as he seeks to remain in the White House until he is 86 years old.

During fund-raisers on Wednesday, Mr. Biden twice recalled a 2021 conversation with Helmut Kohl, the onetime German chancellor, who died in 2017. His spokeswoman later said he misspoke, as many public officials do.

Mr. Biden has tried to laugh off the issue, insisting that with age comes wisdom. And his aides have repeatedly insisted that despite how the president sometimes comes across in public, he remains sharp and tireless when he is in private, in discussions with aides or in meetings with foreign leaders.

But the report released on Thursday challenges those descriptions, not by relying on short snippets of Mr. Biden posted to social media but rather on hourslong interactions with the president in controlled settings. And the descriptions of his memory were more vivid than what is normally found in legal documents like the one released on Thursday.

Mr. Biden’s political rivals, including former President Donald J. Trump, who has had his own string of unforced gaffes, are certain to seize on the detailed conclusions in the report as evidence that he is too frail to lead the country for another five years.

In the report, Mr. Hur wrote that in a 2017 recorded conversation between Mr. Biden and the ghostwriter for his book, Mr. Biden struggled to “remember events” and was “straining at times to read and relay his own notebook entries.” Mr. Hur said that the interviews in 2023 with investigators were even worse.

“He did not remember when he was vice president, forgetting on the first day of the interview when his term ended (‘if it was 2013 — when did I stop being vice president?’), and forgetting on the second day of the interview when his term began (‘in 2009, am I still vice president?’),” the report said. “He did not remember, even within several years, when his son Beau died.”

Mr. Hur was nominated by Mr. Trump to be the U.S. attorney in Maryland, but was later chosen by Attorney General Merrick Garland to lead the investigation into Mr. Biden’s handling of classified documents.

Mr. Biden’s lawyers have been arguing for more than a year that the discovery of classified documents at Mr. Biden’s offices and Delaware home was no more than accidental oversight, and certainly not criminal behavior like the 37 felony charges brought against Mr. Trump for his handling of classified material after leaving office.

On Thursday, the special counsel came to the same conclusion, a fact celebrated inside the White House and at the president’s re-election campaign headquarters, where aides are preparing to wage a fierce battle to prevent Mr. Trump’s return to the White House.

But the report refuted the longstanding argument by the president’s lawyers that Mr. Biden never put the nation’s national security at risk. Investigators found documents at Mr. Biden’s home in a “box in the garage, near a collapsed dog crate, a dog bed, a Zappos box, an empty bucket, a broken lamp wrapped with duct tape, potting soil and synthetic firewood.”

While concluding that “the evidence does not establish Mr. Biden’s guilt beyond a reasonable doubt,” Mr. Hur nonetheless wrote that Mr. Biden took classified documents and notebooks about Afghanistan with him in 2016 after leaving the vice presidency, and shared some of those documents with his ghostwriter.

The tough language by Mr. Hur could set the stage for Mr. Trump and his allies to launch a fresh round of political attacks on Mr. Biden for doing the very same kinds of things Mr. Trump is accused of doing. And it will probably complicate the monthslong effort by Mr. Biden and his advisers to draw sharp distinctions between the actions of the two presidents.

But the most searing political damage is likely to be about Mr. Biden’s age, which many veteran Democrats already believe is the president’s biggest weakness. Some have privately said they worried that something would come along to remind voters about the age issue, including the possibility of a fall or a mental stumble.

Republicans began using the report to attack Mr. Biden almost immediately, sometimes going much further than the prosecutor’s actual conclusions.

Senator Marco Rubio, Republican of Florida, said on social media, falsely, that “the special counsel decided not to bring charges against Biden because they believe he has age related dementia.”

In some ways, Thursday’s report was the worst of all worlds: an official description of Mr. Biden behind the scenes, suggesting that with age come stumbles.