  Lenneberg (1964, 1967) also argued that although language acquisition is remarkably robust, in the sense that all normal (and many  abnormal) children do it, it can occur unproblematically only during  a ‘critical period’ — roughly, up to late childhood or early puberty. On analogy with other supposedly innately specified processes like imprinting or visual development, Lenneberg used the  existence of a critical period as further evidence that language  possesses a proprietary basis in biology.