https://www.politico.com/news/2019/12/11/biden-single-term-082129

This is what he was 'privately signalling' before he was elected.

“If Biden is elected,” a prominent adviser to the campaign said, “he’s going to be 82 years old in four years and he won’t be running for reelection.”

Sure, he didn't pledge it, but his campaign made it clear this was the plan and it's not like any of the age concerns were new if they were talking about them before he was even elected the first time.

But then again, they successfully gaslit everyone about this thing, same as the $2000 check if Warnock and Ossof got elected that turned into a $1400 check (or was it 1200?).