A birthday is a poor screening test. Federal traffic safety officials state the point directly, and families who use age as the trigger for a difficult conversation frequently target the wrong person while missing the driver who actually needs evaluation.
"Research shows that age is not the sole predictor of driving ability and safety," the National Highway Traffic Safety Administration says in its guidance on older drivers, while noting that our ability to drive safely is affected by physical and mental changes that occur as we age, in different ways and at different times.
