The primary source that I’ve seen are videos that show him acting strange on stage, wandering around and talking to invisible people.
If you fact check the videos, they’re clips from public Q&As that have had the audio removed and are zoomed up on him, so you can’t see that he’s having questions called to him from off-stage, that he’s responding to. He’s walking around to look at the different speakers, and he’s talking to them from a great distance, on his mic. It’s just creative editing.
There was one example where he forgot the death of someone whose funeral he attended, rather conspicuously, on stage in the middle of an event:
If he’s not suffering dementia then, certainly, he wasn’t getting enough sleep for a man his age. And, I mean, that’s still something to keep in mind even if he’s a perfectly healthy 80 year old.
Otherwise, the closest that I’ve seen to a reasonable argument is comparisons of him talking, today, to how he talked a few decades ago. He’s definitely slowed down a bit, but that’s not what we’re talking about here.