Norm is 12 weeks and stated as a min. age to keep them to.
Here is their link:
Yorkshire Terrier Club of America Code of Ethics Go down to #8 of Code of Conduct.
It is so much better for the pups - safety/health reasons and for social reasons. I have heard of so many pups being loss if left too early. Check out hypoglcemia information and share with her too. She needs to do her research as a good breeder. Those pups only have her to protect them. Good thing you came here to ask.
T.