The corn will start to sprout off from the main stalk at about 24-30 inches off the ground. At least that's what mine did (Silver Sweet). The plants are around 6-7' tall, presently and I imagine I'm a week from harvesting a piece - maybe a little more.
You could always brace them to be on the safe side. Drive around and find some bamboo growing locally. It's everywhere. If you have only a few plants like I do (I have around 12), you could put a support at each end and zig zag string stalk to stalk from support to support. And that is going cheap. Otherwise, use your imagination and build a fortress around them.
I've noticed something slightly strange on mine. The more mature plants have started another ring of roots about 1-2" above the soil line. They just popped out like boils and then made their way to the soil. Is this pretty typical?