At what age it is good that the kittens are entrusted to their new families?
Responsible breeders give the kittens to their new families at about 12 weeks. Some suggest as a good practice even 10 weeks, although personally I do not think that this is the best thing, it's still an acceptable range.
A kitten aged less than 10 weeks is not fully weaned and socialized, and while we tend to think that "younger is better" and that a larger kitten won't be able to create bonds with the family, in fact this is not true. Cats are not herd animals and are able to forge bonds with human beings at all ages, even from old.
What is true is that a kitten, separated too early by her mother and brothers, may not learn to tighten links appropriately. The weeks between the sixth and twelfth are a very important period for mental development and emozionatale of the kitten. It is during this period that the kitten learn "cat language" (the body language used by other cats), learns to socialize in appropriate manner with his mother and siblings, learn to trust in humans, and develop the ability to cope with the outside world with their forces. At the age of 12 weeks, the bond-cat kittens mother is beginning to come less naturally.
A kitten separated from mother and littermates before this process is concluded could have lifelong problems in interaction with other cats; will never be able to forge emotional bonds with normal humans, they can be fearful, or shy and elusive develop inappropriate attachments to things and objects. In fact, the kittens separated from Mom and littermates prematurely may become "wool eaters", i.e. they suck or chew things like your neck, sweaters, blankets, etc..
Sometimes the kittens that were not quite with Mom are absolutely insecure and become fearful cats who are almost always hidden, if you missed them the necessary period of play with his brothers may develop behavioral problems such as aggression and learning difficulties.
Many kittens, left growing in the normal manner, will attain complete weaning between 10 and 12 weeks of age. Many breeders begin weaning by introducing the kittens feeding solid food sometimes, between 4 and 5 weeks of age, kittens and gradually replaces breast milk with solid foods.
However, weaning is a process, not a single event, the kittens will continue to puppare and continue eating food together until they stop sucking alone or until their mother beginning to be tired of nursing and stop feeding them. The best solution for their physical and mental health is to let the process take its natural course.
More importantly, the period from the sixth to the twelfth week, is a critical period for the health of the kitten, in fact, is the period in which the immune system is freeing maternal antibodies received during lactation to pass immunity given by vaccination antibodies. This is a process and cannot be done in the space of a day.
This period can be a big stress for the immature immune system of kitten. A kitten that is subjected to more stress to be too carried away from familiar surroundings, the mother and her babies can also be more easily susceptible to infections of the respiratory or digestive tract problems, particularly diarrhea.
I certainly wish that your kitten will feed correctly and who knows how to use the litter box when brought home, but your kitten should also be independent of her mom, well socialized and have a strong immune system up and running. The kitten will be ready for her new home, both physically and psychologically when all processes described above are made in a natural way, and this is precisely around 12 weeks.