According to Eric Lenneberg’s Cirtical Period Hypothesis in 1967, the hypothesis theorized that the acquisition of language is an innate process that determined biologically. The notion of critical period was connected only in the first language acquisition (, 2012). Lenneberg assumed that the structural reorganizations within the brain were developed only from roughly the age of two to puberty which was around thirteen or fourteen. Language skills which were neither learned nor being taught during this age would remain permanently undeveloped (Schouten, 2011). Lenneberg’s hypothesis claimed that the absence of language was very limited in the first language acquisition during the early childhood exposure (, 2009). He believed that the brain would lose the plasticity after two sides of the brain has developed specialized functions.The Critical Period Hypothesis is Lenneberg’s response to the long-standing debate in language acquisition over the extent to which the acquire language is biologically linked to age (, 2009) Lenneberg proposed that the ability of brain to acquire a language is stopped at puberty with the onset of brain lateralization. He refers that brain lateralization, which is a process which the both sides of brain develop specialized function, in which after the process, the brain would lose its plasticity as the function of the brain is set.Lenneberg stated that if the child did not learn the language before the puberty, the language could never be learned in a full and functional way. He proves his theory by referring to cases of feral children, such as Genie. Discovered in the age of thirteen and a half in 1970 in an isolated and neglected living condition, Genie did not had any form of communication, and she was neither able to speak nor write. After being saved from her ordeal, she began to learn language slowly, but she never regained full language capabilities.According to Lenneberg, first language learners should receive exposure on their first language prior to puberty for the best acquisition results. He contends that the critical period for learning a first language would same apply to acquiring a second language Studies have shown that before the brain is fully developed a second language can be learned more easily. However, while many people have been able to master the syntax and vocabulary of a second language after puberty, not many achieve native-speaker fluency, compared to first language learners, or bilinguals who start off at a young age. A notable trait for FLL is that their phonological is the most obvious evidence for the critical period hypothesis, as their learning a second language would be impacted by their first language accent.Lenneberg’s works is still highly regarded as one of the most well regarded psycholinguistic argument of language acquisition.