The YTCA isn't the only one that recommends 12 weeks. For those interested here's an article that explains why the 12-week rule is important for small breeds.
Why do Maltese puppies need to be 12 weeks old before going to a new home??. Breeder socialization with the puppy is important as well, and if you are purchasing from a poor breeder who doesn't socialize properly, you may not have received full benefits from the longer socialization process.