Cornsnakes do not care how old they are when they breed. Further down on the forum, there's a post by Dogstar about how two of her snakes bred while she was attempting to take a groupshot. Neither had been brumated, and the female was very young, weighing only 150 grams. It's quite possible her female will not survive laying her eggs, considering the minimum recommended weight I read in the Cornsnake Manual is 240 grams.
I would seriously advise against housing a pair of cornsnakes together, unless you're willing to take the risk that they'll breed far too young and the female could easily die from eggbinding because of it.