The Yorkshire Terrier Club of America's Code of Conduct requires that breeders keep puppies until they are 12 weeks old:
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. Yorkshire Terrier Club of America
I would not purchase a puppy from a breeder who does not follow the breed club's Code of Conduct. It's a huge red flag that they may not be following other rules that responsible breeders follow.