Here is a link to the AVMA (American Veterinary Medicine Assoc.) supporting early socialization for puppies:
http://www.avma.org/onlnews/javma/oct08/081001c_pf.asp

This is the link to the position statement:
http://www.avsabonline.org/avsabonli...ialization.pdf


Puppies should start classes at 7 to 8 weeks of age, after the first deworming, and at least one week after the first set of shots. This dates back to 2008; amazing that so many vets choose to ignore this guidance.