<P> For example, the critical period for the development of a human child's binocular vision is thought to be between three and eight months, with sensitivity to damage extending up to at least three years of age . Further critical periods have been identified for the development of hearing and the vestibular system . There are critical periods during early postnatal development in which imprinting can occur, such as when a greylag goose becomes attached to a parent figure within the first 36 hours after hatching . A young chaffinch must hear an adult singing before it sexually matures, or it never properly learns the highly intricate song . </P> <P> Confirming the existence of a critical period for a particular ability requires evidence that there is a point after which the associated behavior is no longer correlated with age, and ability stays at the same level . Some experimental research into critical periods has involved depriving animals of stimuli at different stages of development, while other studies have looked at children deprived of certain experiences due to illness (such as temporary blindness), or social isolation (such as feral children). Many of the studies investigating a critical period for language acquisition have focused on deaf children of hearing parents . </P> <P> The critical period hypothesis (CPH) states that the first few years of life constitute the time during which language develops readily and after which (sometime between age 5 and puberty) language acquisition is much more difficult and ultimately less successful . The hypothesis that language is acquired during a critical period was first proposed by neurologists Wilder Penfield and Lamar Roberts in 1959 and popularized by linguist Eric H. Lenneberg in 1967 . Lenneberg argued for the hypothesis based on evidence that children who experience brain injury early in life develop far better language skills than adults with similar injuries . </P> <P> Dr. Maria Montessori was one of the earlier educators who brought attention to this phenomenon and called it "Sensitive Periods", which is one of the pillars of her philosophy of education . </P>

When does the critical period of language end