IntroductionHuman beings can demonstrate communication with each other through linguistics and non-linguistic communication. Communication through language can be defined as linguistic communication or verbal communication, while non-linguistic communication or body language refers to laughing, smiling, shrieking, and so on. Most or all non-human species might be able to exchange information, but none of them is equipped with a complex system of communication in which language is introduced as a mean of communication. Basically, they communicate each other through non-linguistic means such as resembling humans smiling, laughing, yelling and raising eyebrows. Take an example, Chimpanzees and orangutans might can exchange different kinds of information by emitting different kinds of shrieks, moving their hands as their gestures, yet they do not exactly produce words and sentences. Hence, human language is categorized as exclusively human property that employs a system of communication based upon words and the combination of words into sentences (Corballis, 2003).Referring to modern dictionary, the term language refers to “the method of human communication, either spoken or written, consisting of the use of words in a structured and conventional way” (English Oxford Living Dictionary, 2018). Different from other creature’s language, human language is unique since it is learnable that consists of certain rules such as morphology, syntax, semantics, and pragmatics. Through language, human can exchange knowledge, beliefs, opinions, threats, commands, wishes, thanks, promises,feelings to others.Moreover, human can learn the way to express it starting from the early stage of their life. For linguists, the learnability of language for non-native speakers facilitates it to be more widespread for the sake politics or business, or even only for personal enjoyment (Alghizzi, 2014). As a result, linguists investigate some factors affect the process of the acquisition of the first language (L1) and second language (L2). They argue to have various analyses and investigations regarding their hypothesis or theories, the Critical Period Hypothesis (CPH) as an example. There have been some historical overviews stating that “the younger the better” for learning language. It is generally accepted for L1 acquisition. However, controversy arises when it comes to L2 learning when the best time to start to learn second language and whether the learner whose begin to learn in early life attain higher levels of proficiency than those whose begins later on (in adolescence or adulthood). Moreover, the widespread of the concept “the younger the better” has strongly influenced in parent’ mind to decide when their children begin to learn their second language or foreign language as well as nation’s policy makers in order to design the school curriculum. Hence, the Critical Period Hypothesis (CPH) for L2 learning does exist, but it is still controversy among the researchers. Nonetheless, this concept gradually influences for EFL learning in some countries as well.2. Theories Concerning the Critical Period HypothesisIt is generally agreed that the ultimate achievement of learning a language is easier to be obtained for younger than older people (Hyltenstam & Abrahamsson, 2003; Nelson & Rosenbaum , 1972; and Scovel, 2000). The question of whether language development is influenced by an age factor still perennially attracts wide interest as well as generates fierce debate. Therefore, many studies regarding age-related language are conducted that leads researchers to formulate some hypothesis on a critical learning period. Before discussing about critical learning period, it might be worth reminding to understand the concept of critical period in the biological sciences. Singleton (2002:5) when commenting on critical period on the basis of biology and first language acquisition states that:”Critical periods in biology can, on this basis, be characterized follows: (1) They relate to very specific activities of behaviours, (2) Their duration is limited within well-defined and predictable termini, and (3) Beyond the confines of the period in question the relevant behaviour is not acquired. If language acquisition in human beings is rigidly constrained by the limits of a critical period of this kind, the implication is that L1 developments begins only at the onset of this period and that unless it gets under way during the period in question it will not happen at all. A further implication may be that even if L1 development begins within the critical period it does not continues beyond the end of that period.”Referring to Singleton’s explanation, it can be said that critical period refers to a predictable limitation time to have an ultimate development and it will stops at the certain period of time. When it comes to first language acquisition, there is a certain period that individuals are not enable to continue their L1 development. With the regards of critical period learning, it is widely accepted that native-like proficiency of language acquisition can simply occur through exposure rather than through learning process or tutoring at ages below critical period learning. At ages above the critical, the language skill acquisition will be much more difficult (Chriswick, 2008). The initial term of critical period hypothesis is driven by Lenneberg (1967) that links between biological factors and language acquisition. The innate process of language acquisition has been influenced by biological factors that limits its time starting from around two years of age to puberty. As it is quoted by Collier (2006), Lenneberg adds that the brain loses plasticity after it experiences lateralization (a process by which the two sides of the brain develop specialized functions). This stage is normally completed at puberty. Hence, individuals on their post-adolescent get more difficulties in language acquisition. However, the exact timing of lateralization remains debatable among researchers. Lenneberg (1967) proposes lateralization happens by puberty, while other researcher Kinsbourne (1975) states that it is completed by birth, and Krashen (1973) suggests the brain gets less plasticity by age 5. Moreover, Long (1988) proposes that there is no correlation between the brain’s loss of plasticity and age since its process is affected by other aspects of cerebral maturation. As a result, Pallier (2007) highlights two different meaning of of the Critical Period Hypothesis (CPH). The first definition states that the first years of humans life is more efficient for language learning while the second definitions clarifies that in order to predict the ultimate proficiency in language learning, the age of acquisition basically plays an important role. It means that the younger individuals start to learn a language, the easier they gain native-like proficiency and vice versa.