I would encourage you to contact the Department of Motor Vehicles in your local area. This is the same office where you would report to get your driver's license. Each state has set laws that govern the operation of farm equipment. You can find the telephone number for the DMV in your county by checking the blue pages of your local phone book.
Here is the type of information you might receive:
No person under the age of 15 can operate any farm tractor or motor-driven implement on the highway unless the driver is on the highway adjacent to land upon which the person lives when that person is actually engaged in a farming operations.
Keep in mind that each state might vary with regard to age and specifications. You may need to ask additional questions for clarification once the law has been quoted to you. Don't hesitate to address the person responding to your inquiry. There is no such thing as an insignificant question or concern when safety or the law is involved.