Laws may be different elsewhere. but here is some laws from ohio
ORC 4507.03
2) No person shall be required to obtain a driver’s or commercial driver’s license for the purpose of temporarily driving, operating, drawing, moving, or propelling any agricultural tractor or implement of husbandry upon a street or highway at a speed of twenty-five miles per hour or less.
(3) No person shall drive, operate, draw, move, or propel any agricultural tractor or implement of husbandry upon a street or highway at a speed greater than twenty-five miles per hour unless the person has a current, valid driver’s or commercial driver’s license.
It also says that EVERYONE who drives a "motor vehichle" is required to have proof of financial responsibility. And here is their definition of "motor vehichle"
(I) “Motor vehicle” means every vehicle propelled by power other than muscular power or power collected from overhead electric trolley wires, except motorized bicycles, road rollers, traction engines, power shovels, power cranes and other equipment used in construction work and not designed for or employed in general highway transportation, hole-digging machinery, well-drilling machinery, ditch-digging machinery, farm machinery, threshing machinery, hay baling machinery, and agricultural tractors and machinery used in the production of horticultural, floricultural, agricultural, and vegetable products.
So.......IN ohio, you DONT need a license if the tractor is operated UNDER 25mph. AND you dont need insurance for said tractor.