any age younger would probably be too risky. personally teenagers shouldn't handle a car cause they happen to use their phones too much and/or care about applying their phrilly makeup. if they focus too much on either of those they can get in a car crash. late 20's would be better.
Cause little people are even worse drivers than big people for the most part. You need to understand the mass and motion and stored energy that are inherent in masses of metal moving at speed. Have you noticed that people die in, and because of vehicles? It takes a certain amount of mental maturity to drive safely and most young people aren't there yet.