How few old people have you interacted with in your life? When you get past 70 or so, there is a ridiculous range of levels of mental and physical health, often strongly correlated with how well off you were/how well you took care of your mind and body throughout your life. Yes, there are plenty of 80 year olds that live completely independently. I'm sure there's even been 80 year olds who ran marathons or climbed Mount Everest.
But on the opposite end of the spectrum, there are also 80 year olds that are effectively infants, shitting in diapers and not being able to speak or recognize even their own family members. Alzheimers and dementia are horrifying diseases and I kinda envy the charmed life you must be living to have never interacted with an older person in late stages of those conditions.
You can often get a good idea of how someone is going to age by looking at their parents and grandparents. Of course there are exceptions, like if your grandpa was an NFL linebacker or worked in a lead smelter, genetics probably didn't cause the dementia. Or if they got killed by something else, like cancer, accidents, or war, before they could reach the dementia age.
Neither of Barb's
parents reached 60, but they didn't seem like healthy people.
Meanwhile, Bob's final years give an excellent glimpse into Mild Cognitive Impairment:
Many people use the infamous prank call where someone got Bob to argue with a soundboard of himself as evidence that he was gone. But most people's own voices sound weird to them when played back.
Dementia: forgetting what country you live in.
MCI: being pretty sure that Molvania isn't a place, but mixing it up with Moldova because you don't know how to check Google.
If Bob had just stayed away from Barb, he would've had a pretty good old age, instead of dying covered in mites while his son raged at imaginary people upstairs.
Chris is almost certainly aging like Barb.