You're dismissing the obvious - or is that just cynicism? Biden was slaughtering and mashing up words before he became president. "true and internash of a depressure"
Here's Reagan's farewell address a few weeks before he turned 78. Do you recall how the legacy media painted him as having Alzheimer's/dementia during his second term? I was a young man at the time and went along with the media's narrative, though it wasn't called that at the time.
"...anyway."
Here he is at 83:
No, I do not recall the media claiming he had Alzheimer's/dementia while still in office. As you point out, a lot can change health-wise in 5 years when you’re old (I know, I am old

). The difference between these two speeches is a clear indication of this, at least for Reagan.
I am not sure what the point is that you are trying to make and why. If elected, Trump will be the same age as Reagan was at the end of his two terms. Biden is 4 years older than Trump. Both men are too old to be President of the United States, as I have now said so many times, I am getting 'blue in the face."
As I see it, here are the two possible choices of who to vote for come November. There is Trump who appears to suffer from mental illness, most likely narcissistic personality disorder and thus is at the least a sociopath and possibly worse a psychopath. These are not conditions of age. Anyone at any age can have mental illness. Then there is Biden, who might have dementia and if not could by the time he leaves office in 2008.
To be honest, Trump's mental illness concerns me more than Biden possibly being senile. But like I said and will continue to say, neither of them is fit to be President IMO.
A couple of things might happen between now and November which could improve this awful situation. One is that Biden may fall ill as sometimes happens when you are an octogenarian and will be forced to bow out requiring the Democratic party to produce a more viable candidate. Or two, Trump completely loses it to the point that even MAGA folks finally realize how mentally sick he is (unfortunately MAGA folks waking up to this truth is highly unlikely). Another possibility is Trump is disqualified from running because he is convicted of one or more of charges against him that are felonies. Lastly, I suspect Trump is physically unwell or at least a good candidate for a major setback which takes him out of the running. This is not so far-fetched because stress is known to cause major physical health issues, such as a stroke or heart failure, (not that I wish this on him or anyone for that matter).