Pedigree kittens are indeed ready to leave for their homes at 13 weeks old. However, non-pedigree kittens can leave at 8 weeks if the owner is happy with their health & growth etc...The 13 week rule is to ensure that the second vaccinations have not caused an adverse reaction to the kittens.

Also bear in mind that in many other countries including the US, pedigree kittens actually leave at 9 weeks old after their FIRST vaccinations, and reputable breeders have obviously been happy with this for many years, and the cats haven't suffered as a result of being homed at this age.

I think common sense plays a part with non-pedigree kittens. If the kittens have been well cared for and they have been brought up on a healthy diet, then 8 weeks old can be a safe age to home them.