What is the age limit on owning a semi truck?

I don't think there's an upper age limit. As far as lower age limits go, you've got to be 18 to drive one in intrastate commerce and 21 to drive one in interstate commerce. I cannot think of a service in which an 18-year-old could drive--at first I thought "guys hauling milk from the farm to the bottling plant" but milk haulers need a LOT of experience because milk trucks are dangerous. What happens is the milk bunches up at the back of the trailer. When you step on the brakes, it all goes to the front of the trailer. This is called surge and it can push a truck out into traffic. But to own one? If you were a really rich kid--like Miley Cyrus or the Jonas brothers--you could buy a semi truck at the age of five and have your parents hire a driver.