I'd say it should differ between rural and urban areas. In urban areas there are quite a few distractions, more motorists on the road, and pedestrians. In rural areas, you won't find those issues. Urban areas also have more options when it comes to mass transit.
I know for my home state, Minnesota, they have laws for this. You can get your normal license at 16 but if your family are farmers, and you meet certain criteria, you can drive by yourself after completing a drivers education course at 15. You can also drive licensed farm vehicles without a license at I believe 14. This is mainly tractors and farm equipment but you can get a truck registered for farm use only.