Why do breeders keep the kittens until 12 weeks of age?
It is best practise for several reasons. The kitttens need the support and care of their mother for the first weeks of life for their development and socialisation. The breeder also needs to watch the development of the kittens very cosely to determine their quailty and to handle and nuture the kittens for human contact. Kittens need the company of their litter mates also to play and interact. Finally but just as importantly, vaccines cannot be given to very young kittens. Vaccines for kittens (primo vaccination) are given in two parts up to 2 weeks in between.
Note. The law in France states that kittens may not be released to new homes until they are at least 8 weeks old.