I vote 12 weeks. Puppies can learn a lot from their moms and siblings in that time. Also, Yorkies are tiny to begin with, so the extra 4 weeks gives them time to grow some more and become stronger.
I'm not as familiar with larger breed dogs and whether or not they are kept by breeders until 12 weeks.
I brought home my second boy, Teddy, at 8 months and like to joke that I wish I had left my first one with the breeder that long. Teddy was perfectly trained at 8 months.
