My vet has always told me she can't estimte yorkies age so well because they get adult teeth at different times, not all are the same. When I got Whisper she was going on 6 months and had all her baby teeth in still. She didn't lose them til about one to 1 1/2 months later. (I think can't really remember) You should have a talk with her breeder and ask her question let her know what your vet suspects.
Smiles,
Myra