I can tell you that Biden has more energy and is more with it for his office than I am in trying to resume my profession after my stroke. For him, he looks and acts like someone healthy for his age. He stumbles over words so he doesn't stutter, that's apparent when he talks. He might have mild aphasia, which occurs when under stress or fatigue.
When I write, I can hide my aphasia. When speaking, it's more apparent and I pause while I attempt to find the word. And I'm much more literate when writing because it was my profession. I'm also subject to loss of stamina that happens suddenly. I've become more cognizant of the symptoms of losing stamina. It makes visits and in person interactions and phone calls shorter and I inform people that when the symptoms appear I need to absent myself from the interaction. It mostly comes on very suddenly. I get quiet and lethargic.
I'm 72.
BTW, Trump trying to pass off his speech foibles as deliberate is disingenuous along with his other dises.