hmmm ... if the snake is 18" my personal guess would be that his age is around a 11-14 months. That is unless they were feeding him crickets and such then he would probably be underweight and thus length anyways.... anyways I suggest the same feed an appropriately sized prey item (as Gen suggested). I too suggest switching to Pre Killed (P/K) and or Frozen Thawed (F/T) mice as they are safer, more economical, and easier to keep on hand. I would say feed once every seven days for the next two years or so - basically until the snake appears to have reached adult size (length and width) which of course differ between females and males. Glad you're here!