A 3-year-old could start learning to drive a car if the controls were adapted to their physical size. Driving a car is designed to be easy enough for anyone to do it, and kids routinely learn more difficult skills. The main reason we don’t have kids driving cars is because of poor decision making and impulse control, not poor reflexes or untrainable motor skills.

If you want to teach a kid to drive a car you should probably stick to video games or low-speed go karts though.