My father slid into age-related dementia. At age 92, he was still driving (and getting into numerous fender-benders). He exercised poor judgment. He decided not to pay his bills, despite prompting. (He said he'd let his estate pay the bills.) He'd watch FOX News when he wasn't asleep and then parrot whatever he heard, like a tape loop. It was hard to be patient with him. I moved back with him to keep an eye on him.
My BFF from college (age 67) started to decline about two years ago. I noticed unusual remarks and habits. I consulted with her sisters and they confirmed they saw it too. Her predominant feature is her loss of memory. Some things she can remember; other things, not so much. She can no longer live independently and her sisters have moved her into their family home with an aide. And yes, she is aware that her memory is a problem. She now sits in front of the television, watching sports, but not taking it in. Like a nursing home patient. She seem unperturbed by her situation generally. She has been caught trying to walk one mile away to her childhood home, thinking her family still lives there.