It is true that people who have always communicated "properly through language" can lose that ability. Because language is a function of the brain, when something happens to the brain, the ability to communicate through language can be disrupted or eliminated.
As many as eight million Americans suffer from some kind of language disorder. Language disorders can be related to the form, the content, or the function of language, but language impairment is often different in children and adults.
Children's language disorders are generally related to language acquisition, whereas language disorders in adults are generally caused by some kind of trauma to the brain. Such things as brain tumors, dementia, strokes, or head injuries can cause adults to lose their ability to communicate effectively through language, despite the fact that they were once able to do so.