Like others have said, it all depends on the laws of your state and the states where you will be hauling.
That being said, since I grew up out in the boonies, it seems that I started most things younger than other folks.
I started driving around the farm when I was around 5 or 6 (had several minor accidents...running into fences, putting dents in the barn, etc) but I learned how to drive and drive well. I took driver's ed when I was 14 and got my learner's permit the day I turned 15. That same day, I started driving the pickup and the 20 foot gooseneck that we would use to haul horses and dogs to check cattle. Of course, only having a learner's permit, I had to have a licensed adult with me, but I started hauling by myself when I was 16 and got my real license. I've never had an accident with a truck and trailer, though I've come close several times due to the stupidity of other drivers.