In Chapter Two (How Psychologists Do Research) one of the case studies was about

Genie was locked up in a small room strapped to a potty chair from age 1 ½ to age 13.
Her mother, a battered wife, barely cared for her.
No one in the family spoke a word to her.
If she made the slightest sound her father beat her with a large piece of wood.
When rescued, she did not know how to chew or stand erect and was not toilet trained.
Eventually she learned some rules of social conduct and began to understand short sentences.
She used words to convey her needs, describe her moods, and even lie, but her grammar and pronunciation remained abnormal.
Study of Genie supported the idea that there is an early critical period for learning language.
The likelihood of fully mastering a first language declines steadily after early childhood and falls off drastically at puberty.