That is a good question but to drive a big truck interstate, here in the U.S. you have to be 21 and that is federal regs. I would have to assume that no you can't drive the big truck across the border, but if anyone knows differently please jump right in.
I think you're right that the minimum age is 21, although a few years back there was a pilot/test program for younger drivers; I have no idea whatever came of it.
I also believe that Canade has a law that anyone with a DUI on their record is not allowed to drive into Canada; I may be wrong, but I've heard it a few times. Just some extra info for those thinking about being a trucker and have a DUI on their record.