I can only speak from my experience with my own mother.
About 10+ years ago in her late 70s/early 80's she got lost driving herself to her hair appointment. An appointment in her neighborhood where she had driven herself before many times.
She informed my sisters and I that she was going to give up her drivers license and stop driving. She thought she was becoming dangerous.
We chalked it up to old age and the onset of dementia. Little did we know that she was on the path to 7th Stage Alzheimer's disease.
We drove her where she needed to go, and thought that the driving issue was behind us.
Several years later, she wanted to go to a place that no longer existed. We told her what was going on, and it made her frustrated and angry. She denied what we were telling her, and informed us that we could piss-off and she would just drive herself there no matter what we thought. She couldn't find her keys... we had taken them for keeps.
When we told her that SHE had told us not to let her drive anymore, she accused us of lying. When we told her that she did not have a valid driver's license or insurance, and that if she got pulled over she might be arrested, she broke down crying.
In our family's experience, there was a point where she realized her limitations and wanted the best for everyone concerned.
And then there came a point where she didn't anymore.
It's heart-breaking to witness, and I don't wish it on anyone.
But there comes a time where reality smacks you right in the face.
Peace