Let me play in this one too. For a normal POTUS the demands of the job can be overwhelming. Not for Trumpty though, because he does little. His "executive time" takes up half the day, golf takes up his weekends and then there his "tweeting time". And clearly he spends no prep time for his meetings with other leaders.
I would say that yes, whatever form of dementia he has- it is progressing. However- there is also the fact that he is- by our standards seriously under educated. We know he didn't do well in school and he us actually very little smarter than his Uneducated followers. I'm betting he has no idea where Mars actually is located. Today's wonderful history lesson just fell out of his mouth without anything but his fat head to back it up.