Studies show that, as a group, older drivers drive less than younger drivers, but they have more accidents per mile. Unsafe drivers who continue to drive despite the advice of family and friends often do not come to the attention of the state until the inevitable -- the driver is stopped for erratic driving or, worse, he or she is involved in an accident. A few states try to screen out unsafe older drivers by requiring more frequent written tests. But the added tests don't always identify unsafe driving habits.
All licensing departments accept information from police officers, families, and physicians about a driver's abilities. If a licensing agency moves to cancel someone's license as the result of an officer's observations, an accident, or the report of family members or a doctor, the driver usually has an opportunity to protest.
For more information on drivers' licenses and traffic tickets, see Beat Your Ticket: Go to Court & Win, by David Brown (Nolo).