My own opinion is that BOTH Biden and Trump are more-or-less as fit mentally as they were a decade ago. Biden has minor lapses, but we all do, especially when old.
There are YouTubes where a psychologist claims that Trump suffers from dementia, but when he's speaking off-the-cuff he seems like his old stupid self. Some of his slurring might be due to minor brain damage. But the inane rambling was his symptom a decade ago. The drifting-off gibberish seen on anti-Trump videos is, I think, caused by his attempts to read from a teleprompter -- He's only semi-literate.
I disagree in that I heard recordings of Trump speaking in public, perhaps the Johnny Carson show?? And while he was and always has been a vile, self aggrandizing blowhard. If you squinted you might be able to hear him as somewhat of a charming blowhard if self aggrandizement and being a blow hard was attractive to you. Now, frequently he does not make sense, even with generous latitude granted for the inevitable mis-speaking and confusion of names and places and even facts that are seen in most tours even of younger people. Yes, sometimes they seem to get his meds right and he is semi-articulate.
Both men are pribably past their intellectual primes. There are two major differences: Biden is bad call a good, if flawed person. Trump is morally corrupt and morally bankrupt and of at best, medium intellectual capacity in his prime. Trump seems much older than his age. Biden seems pretty good for his age and in fact reminds me of the handful of relatives I knew who were pretty sharp well past his age. If he remains healthy, I think he’ll make it to 100, mentally sharp.