Socialization for Puppy Slash Adolescents

Emma wrote, could you share some thoughts about socialization for puppy slash adolescents who are past their socialization window? I've recently taken on a very fun little Boston Terrier puppy. She is five months old and her breeder held her back as a show prospect but determined she would be better suited to a sport home. Emma later added that she did return the puppy to the breeder because the puppy seemed to be sensitizing to new environments despite repeated positive exposure.