Most animal behaviorists recommend against adopting a kitten before 12 weeks of age. Kittens need that time to develop socialization skills with their litter mates and mother. Also, reputable breeders don't like to release them before that point. This affords them time to begin the kittens' immunization series, perform different hands-on exercises to prepare them for their eventual home, as well as early trips to the veterinarian.
For more information, read the story How to Pick the Right Cat For You