Lots of us thought he was too old four years ago, and haven’t changed our minds since then. The question isn’t about if most voters think he’s too old (they absolutely do), it’s about where all the headlines have been the past four years, rather than suddenly after a debate that the vast majority did not watch; "Though polling has consistently registered massive public concern with Biden's age and his ability to withstand the rigors of a high-stakes campaign, let alone another term in office until he turns 86 years old, the loudest voices trying to force him out of the race are elites: major media columnists and wealthy donors” according to David Dayen, and I agree.
Had the DNC held a real primary, you probably would have seen voters revolt, too. I know you're going to say "There was a primary" but there actually wasn't. Florida and North Carolina among several other states literally didn't allow one, there was literally no vote at all. Other states, like New York, don't allow write-ins (or "allow them" but they don't count them) so on an NY ballot in the primary, for example, there was literally one checkbox for Biden, because they kept Maryanne and Dean Philips off the ballot, like it's a vote in Hong Kong. None of the other candidates in the primary were allowed on the ballot in enough states for it to have ever been possible for them to win. If that's an election, Russia and China and Iran are democracies.
We saw the debate. Trust me. It was ripped to shreds into digestive little bites across every smoldering remaining heaps of social media that made everyone sick to their stomach.
And the reason you didn't hear about it before is that Biden has only done 140 press meetings since taking office which is on track to be about half of any president in the last 40+ years.
We didn't see him till then but he was seen. And it was bad. No amount of trying to twist reality will change that so we have to look at it really as it is and start making plans or not look up till the end.