Being fair, I'm of the opinion that both Joe Biden and Donald Trump suffer from cognitive decline in separate ways. However, textbook wise neither man may fit the bill and just be old. Here is the definition of cognitive decline:
Cognitive decline refers to a gradual decline in various aspects of cognitive function, such as memory, attention, reasoning, and decision-making. It is a common phenomenon that occurs with normal aging, but can also be a symptom of underlying medical conditions.
Biden: We don't have documented, on record statements of the man having a lapse of memory, reasoning or decision making. Yes there are anecdotal stories coming out of those who won't go on record, but if you listen to Joe speak he can recite short and long term memory. Where Biden represents "CD" is his ability to be an effective public speaker. He messes up names, says wrong facts, etc but he did that 20 years ago. It gets highlighted now because he's an old man and looks old AF with the gray hair and no makeup.
Trump: Again being fair, what I've noticed is Trump seems slur his works a lot now. Just listen to when he says 'States' as in the United States of a America. Maybe he wears poorly fitted dentures, idk! Trump also has a severe loss of his vocabulary from 20-25 years ago. He uses words like 'stupid' or 'dumb' or any other demeaning simple adjective to describe people and things like a pre-teen. This is free speaking Trump and not teleprompter Trump. The last thing I noticed is we know Trump embellishes from years of suffering from PNSD, but even his stories change and the "facts" change from story to story. Trump doesn't look as old as Biden because he's plump and wears makeup and bleaches his hair. At leas the has that going for him.