Puppies have a «critical socialization period» for learning that lasts until the puppy is approximately 16 weeks of age. (jhpositivetraining.com)
Twelve weeks leaves only two weeks of critical socialization time — assuming there's a class starting up immediately after the pup receives his second shot. (whole-dog-journal.com)
They missed out on critical socialization as puppies and may have little to no concept of living in a home. (talentedtails.com)