You seem to be hung up on weight? I've bred cornsnakes at 9 months of age. Nature doesn't have a scale. Husbandry is the key word here. If you want to keep a single animal as a pet and not let it be a cornsnake then yes you can weigh it and feed it a single prey item every "insert calendar date" here. Her original question was what a 5 month old corn should be eating. A small mouse is no problem for a properly maintained cornsnake.