Dogs, like children, have a critical learning period. Once past this age, socializing a dog can be much more difficult, so starting young is essential. The socialization period for a puppy ends at around 12 weeks of age. This doesn't mean that your job is done when they hit 12 weeks, but that the four weeks between bringing home your puppy and the end of their socialization period is the most critical time for them to be exposed to new things, noises, situations, and people.