One grandmother of mine said she would stop whenever we asked her to. The decision was made when she was 93, and she hasn't driven since, and is now 95. My other grandmother however, rest her soul, had the keys forcibly taken at 80ish and she would still find them and sneak and drive...we stopped keeping keys in her house and then and only then did she stop under extreme protest. The whole process was finished eventually with one accident involving another car and both were totalled, and another where she met a pine tree at 45 mph, which totalled another one...she survived both, to be diagnosed with dementia a couple of years later, lived till she was 87.
I think drivers tests should have to be retaken at 65, and every 5 years after that till 80, then every year till no longer able to drive. Might be extra burden on the highway department, but if it saves lives then it's worth it.