The vet (who should know these things) tell me that there is NO difference at all if the kitten leaves at 9 weeks than if it leaves at 13 weeks - I always thought they should leave at 13 weeks but i did think the vet had a point. I'm not saying my kittens will leave earlier than 13 weeks i just wondered what other breeders opinions are and why you wouldn't allow them to leave early if it does the kitten no harm.