Let's face it, this is an age issue. For sure, not every 70 or 75 year old is demented, but it's a fair point to raise if someone that age is POTUS. That's also why Biden shouldn't be prez. Possibly Sanders as well, although he makes a more vital impression.
Trump has definitely lost some of his cognitive skills in recent years. Just compare his interviews now with from 20 years ago. Back then, he had a much wider vocabulary. Today, he's talking a lot of crap, regardless of his actual policies.
We had Reagan, and he descended into senility while President. At the least he had good people around him capable of monitoring the effects of a Reagan proposal and getting him to back off when something went awry. The Veep was often doing the more complex stuff as the President became more suspect. Donald Trump surrounds himself with yes-men, and the Veep is simply a fanatic who will differ in style and emphasis alone should Trump die or go into a permanent vegetative state. The Trump Administration has filled itself with yes-men who simply obey orders.
Whether sane and immoral or simply demented, we get the worst from Trump. He lacks the judgment to recognize that a competent Presidency following that of Barack Obama does not depend upon repudiating him and offending the sensibilities of people who think Obama at least good, let alone above average.
It is more than an issue of age; it is also an issue of character.