It's not necessary to conduct a clinical diagnosis of dementia to notice that someone is in decline. I certainly noticed it in my mother (who did in fact eventually die of dementia) years before she reached the state where it was an actual medical problem. Now that I'm elderly, I notice it in myself. I'm not demented, but I am forgetful, I struggle to remember names and events that I would have known immediately just a few years ago, and I have trouble concentrating hard enough to understand difficult arguments that I could have analyzed (albeit with some effort) in years past. I know that many people remain sharp as a whip all their lives, but most people slow down mentally, and it doesn't take a medical exam to find evidence of it. The symptoms are apparent. As for Biden, I well remember his rambling "questions" of witnesses at hearings that went on and on for several minutes when he was a Senator, so that's nothing new, but his fumbling over words and sudden changes of thought mid-sentence do seem to be more frequent these days than I remember their being during the Obama administration. As for Trump, I didn't pay any attention to him prior to his presidency, so I really have no long-term baseline for comparison.