The Yorkshire Terrier Club of America requires that puppies be at least twelve weeks old before they are sold. If a breeder tries to sell you one any younger, it's a huge red flag that the breeder may be violating other YTCA rules and it's best to move on.
8) All puppies leaving the breeders possession will be a minimum of twelve (12) weeks
of age to facilitate adequate socialization as well as appropriate emotional and
temperament development through interaction with siblings, dam and other dogs.
. http://www.ytca.org/frame_index2.html