No, I don't believe Joe Biden has Alzheimer's. It's a clinical disease, and I have seen no medical expert speculate that he does. Dementia? Perhaps some early-onset/signs, like many 78-year-olds. We've seen him have issues with memory, language, thinking on the fly, etc. Again, he's 78 years old. He was 18 when JFK was elected. That's pretty fukking old. He's not even a Boomer, he's from The Silent Generation. There's no shame in getting old and having your cognitive abilities degenerate, it happens.
I think looking at average life expectancy is a bit of a red herring for two reasons:
1) The topic of discussion isn't whether Joe Biden is going to croak soon, it's his mental faculties. Unless you're comfortable with the idea of a warm body being President, his mental health matters as much as his physical health.
2) The average 78 year old man isn't about to embark on one of the most stressful, mentally taxing jobs on the planet, so using the average metrics doesn't really apply here. The Presidency ages the fukk out of people, and Biden doesn't much headroom.
We don't have to defend the gerontocracy.