From my recollection, people who fail the vision part of the driving exam get told to see an eye doctor.  It is up to the doctor to determine whether the problem is vision-related or cognitive, and whether it ought to keep the person from driving.\n\nIn the case of people who are legally blind or otherwise can't drive, the government does issue non-driver identification cards that can be used like a driver's licence for ID, but not for driving.\n\nIf the eye doctor feels a person failed the vision part of the driving exam because of a temporary problem (an eye infection that affected vision temporarily, for instance), they can write out a pass that basically says "This person passed the exam at my office" for them to bring back to the DMV.  Most of the time, though, it's left up to the doctor to explain to the 70-year-old why their eyesight is too bad for them to drive anymore.