This is the best explanation of the 12 Week Rule I have found. It's from Foxstone Maltese, but applies to Yorkies as well.
Why do Maltese puppies need to be 12 weeks old before going to a new home??
The best takeway message from this is not to get a puppy from a breeder who will let a puppy go before 12 weeks. This breeder is not knowledgeable about the breed. It is like playing Russian Roulette with the future health of your puppy if you get one from an inexperienced breeder.