While I am certainly no expert, although I've been around the occasional elderly relative with dementia, he's had this pattern of lying for as long as anyone can tell. It's not at all anything new.
I've also on occasion known someone who can be characterized as a pathological liar (not sure if there's such a diagnosis, but there's the definition out there) who lies compulsively about nearly everything and doesn't seem completely aware he's doing it. I've also known people who lie about things (like being able to obtain tickets to a Superbowl game) that are really stupid to lie about, but they do it anyway.
I'm sure most people here have likewise known people like this.
People with dementia, so far as I can tell, don't lie about stuff. They can't remember things, and may go through a time when they will make up stuff, but they don't do it in anything resembling the way Trump does. For him, reality is created by what he says. I suspect he's the sort of narcissist for whom others are not completely real. A bit like someone who is very autistic, except he's obviously not at all autistic.
There also seems to be reason to think he's functionally illiterate, or, even if he has good reading skills, he never reads anything. Possibly not even the contracts he signs, let alone books, newspapers, or magazines. What information he absorbs comes to him through Twitter and perhaps some other social media and what those around him tell him. The concept of fact-checking is totally alien to him.
If he had dementia he'd probably be easier to deal with. It seems fairly obvious that Reagan was well on his way into Alzheimer's the last years of his administration, and everyone around him simply protected him and spoke for him. Trump has no problems speaking for himself, and he has nearly zero acquaintance with truth or reality.
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