Males can breed earlier than females.
With females (and males really) its not about age its about size. A female corn should be at LEAST 36 inches with really good girth. My female was about 45 inches long, and thick before I breed her.
Another thing is the more time you give your female to mature, the larger your first clutch will be. Smaller females= smaller clutches generally. So its even better to wait an extra year.
Pick up: The Cornsnake Manual by Kathy Love.....its a great book on corn genetics, breeding, care, everything.