What we recognize as “socialization” occurs during sensitive periods as the puppy’s brain develops. Although it's important for dog owners to socialize their puppies, the socialization period actually starts earlier than a puppy would be allowed to come home. That's why it's important to select a responsible breeder who will expose the puppy to different sights and sounds to prevent them from developing fear. Here is how socialization affects a puppy early in life:

Birth to 13 days: The mild stress of daily handling is beneficial and allows pups to better cope with later stress.
 Breeders are skilled at doing this while the puppies are young without causing fear or trauma. 

Day 13 to 20: Puppies become more coordinated, open their eyes, and may be startled by sounds. Positive interaction with people during this time is important. 

3 weeks of age: If puppies are exposed to someone who is passively observing them, they will approach and explore that person.
 Positive situations during this time will encourage the development of a friendly, confident dog. 

5-7 weeks: Fear of humans and other species, and fear of the approach of humans, appears if dogs are not socialized. Introducing them to new sights, sounds, and smells with positive associations (praise, treats) will help eradicate this fear. 

Once you bring your puppy home at 8 to 12 weeks, he will be well on his way to becoming a confident, friendly dog. Enrolling him in the S.T.A.R. Puppy program can help you continue him on this path. 

Puppies isolated from humans through 20 weeks often become fearful of humans, are timid, and have impaired learning ability.
 Puppies kept in kennels with no human contact beyond 14 weeks often become shy and demonstrate a lack of confidence in any situation other than the kennel. 

But if you acquired a dog later in life, that doesn't mean he's a lost cause. Training for the AKC's Canine Good Citizen program is an excellent way to help shape those social skills later in life. Even pit bulls previously used in fighting rings are among the thousands of dogs that have become trained enough to earn the title. It's never too late for your dog to become a good citizen.