I'm a neuroscientist. Our candidates have shrinking prefrontal cortexes

Even if the stats predict he is not, stats are only a prediction and it cannot be assumed to be true for everyone. So we cannot say this with 100% certainty he is not as sharp now as he was at 30.

Here is one way to test this: compare the written works of aged authors compared to earlier works. how much of a decline is there in style, word choice, plot , etc.?

Do people not know history? America has had at least one president run the country while in a coma! If there's one thing the American system is actually pretty good at, it's keeping the lights on while the executive person themselves is somewhat indisposed. Presidents are primarily figureheads of their administration, and that administration can easily keep working even if the figurehead is taking a brain damage nap. In fact, I'd rather we work to make such a system MORE transparent and obvious. People should understand that the president themselves are not that essential, which is part of the reason why our list of "next in line" when the president dies in office is so deep, even though a lot of those people in line are not typically treated as if they are just a few deaths away from "running the country". There's very few things that actually REQUIRE the president to be involved. The president is also immensely less important when half the politicians in the country explicitly make "don't let ANYTHING happen" their official policy goal.