The Importance of Early Socialization
Socialization involves introducing your puppy to a wide variety of people, other dogs, sounds, surfaces, and anything else that you'd like him to be comfortable around as an adult. From the age of approximately 7 - 12 weeks, your puppy is more accepting of new and different things than he will be at any other point in his life.
As your puppy passes 12 weeks of age he will gradually become more and more cautious of the strange and unusual. Dogs that are socialized as puppies will adjust more quickly and with less stress to unfamiliar things than those who are not adequately socialized. This is why early socialization is so important.
A lack of socialization often results in fearful, reactive behavior while well socialized puppies grow into well-adjusted adults. Older puppies and adult dogs can still be socialized. However, because they are no longer as accepting of new things, it will take longer and will be much more work on your part.