Biden’s age was a bigger factor because it was absolutely clear he was in rapid cognitive decline. No doubt it’s also happened to Trump at his age (as it does to everyone), but Biden essentially hid from any unscripted press the entirety of his Presidency.
Trump does the opposite. We can all judge his actual cognitive faculties because he’s constantly tweeting or in front of cameras. It’s pretty clear to anyone with a brain that most of the things the Biden administration did, Biden himself had very little to do with. Trump, again, is the opposite.
The NYT honestly should have been covering Biden’s decline more. It bordered on a coverup. The fact that people were surprised at his debate performance points to that.