The people closest to President Biden were well aware that he had changed. He talked more slowly than he had just a few years before, needed to hoist himself out of his seat in the presidential limousine and walked with a halting gait.
“Your biggest issue is the perception of age,” Mike Donilon, the president’s longtime strategist, told him in mid-2022, according to three close aides who heard it. That bit of feedback, delivered repeatedly by Mr. Donilon, was the sort of blunt talk that did not often make its way to a man who had spent a half century in politics prizing loyalty and deference.
Mr. Biden acknowledged the concerns, but the warnings only ignited his defiant, competitive streak. In April 2023, without convening his family or having long deliberations with aides, he announced he was running again.
Now, as President-elect Donald J. Trump heads back to the White House, demoralized Democrats debate what might have been had the president bowed out in time to let a younger generation run. Mr. Biden, 82, has at the same time made the extraordinary admission that he might not have made it through a second term. “Who knows what I’m going to be when I’m 86 years old?” he said in an interview with USA Today on Jan. 5.
The president’s acknowledgment has put a new spotlight on his family and inner circle, all of whom dismissed concerns from voters and Mr. Biden’s own party that he was too old for the job. And yet they recognized his physical frailty to a greater degree than they have publicly acknowledged. Then they cooperated, according to interviews with more than two dozen aides, allies, lawmakers and donors, to manage his decline.