Welcome to BYC. Glad you decided to join our flock. Pullets can be fed layer feed from the 18th week on, even if they aren't laying yet, so your pullets are definitely old enough to be eating layer feed. Cracked corn is okay for an occasional treat, but treats should make up less than 10% of your bird's diets as too many treats will make them fat and unhealthy and will reduce their lay rate when they do start laying. Starchy treats like cracked corn is the equivalent of us eating candy bars and potato chips. Commercial feeds such as layer feed are specially formulated to meet all a chicken's nutritional needs. Please feel free to ask any questions you may have. We are here to help in any way we can. Good luck in getting those eggs.