Biden administration aides have described selectively edited footage of Mr Biden’s gaffes as Right-wing “conspiracy” videos and denied that he has any issues in private.
A flurry of speculation prompted by the February publication of special prosecutor Robert Hur’s report, which said Mr Biden forgot when he served as vice president, was quelled by his strong performance at the State of the Union address in March.
Although Mr Biden’s age has waned and waxed as a topic in the US media, discussion of him being replaced remained taboo in the liberal press until Friday.
Ezra Klein, a New York Times opinion columnist, was vilified in February for writing that there may be a “contested convention” where Mr Biden is challenged to step aside.
The White House has taken to attacking media outlets that report on Mr Biden’s age, including in a Wall Street Journal story that included quotes from officials worried he was “slipping”.
Emma Tucker, the newspaper’s editor, said she felt vindicated about the story after Friday’s debate.
“The reporters took a lot of grief for covering a story that needed to be covered and that no other mainstream publishers were willing to touch,” she said.