A major issue in the field of the CA hypothesis is the difference between children and adults in language acquisition. Symptoms of traumatic aphasia (direct, structural and local interference with neurophysiological processes of language) that occur under age 13 are reversible, whereas those that occur after 13 are not. Non-deaf children of deaf parents who are exposed to a normal language environment at school age learn to speak within a year; deaf persons who reagain their hearing after puberty never master a spoken language. Lateralization of brain function around the age of puberty seems to be the physical correlate of these phenomena. After puberty the brain becomes, as it were, less plastic and therefore less able to take on certain kinds of new tasks.