Later, linguists began to focus this term particularly on its implication on language development of children. Now the term is basically in a linguistic sense when people saying or discussing. This theory is extremely contradictory and argument-generating that there are numerous and uncountable literature on both sides, support or negate, and thus it is also known as critical-age hypothesis, or critical period hypothesis. And within one side of opinions there are also endless debates on more detailed questions. Although the opposite arguments are still vivid today, the basic idea of critical period theory is widely accepted.

The critical-age hypothesis assumes that language is biologically based and states that the ability to learn a native language develops within a fixed period, from birth to middle childhood. During critical period, language acquisition proceeds easily, swiftly, and without external intervention. (Fromkin, Rodman &Hyams, 2007:53)源'自-吹冰;文,论`文'网]www.chuibin.com

Based on this hypothesis, it is reasonable to claim that the earlier children begin to study a second language, the better effects will result. Kindergarten is the first formal institute that a child is going to receive basic education, prepare for elementary school and get socialized by interaction with teachers and classmates.  It is the optimal time that she can develop competence of a second language in a institute setting by spending 3 years before her language function develop to a more fossilized level. 

One exceptionally notable contribution is Krashen’s differentiation between to acquire a language and to learn a language as he calls it The Acquisition-Learning Distinction:

“The first way is language acquisition, a process similar, if not identical, to the way children develop ability in their first language. Language acquisition is a subconscious process; language acquirers are not usually aware of the fact that they are acquiring language, but are only aware of the fact that they are using the language for communication. The result of language acquisition, acquired competence, is also subconscious.” (Krashen, 2009:10)

Inspiration could be found in the distinction if we try to guild the young children to acquire the foreign language instead of learning it in a rigid way. At present, English presenting as a subject begins at the 3rd grade in public elementary school. This arrangement shows both the affirmative concern, that children should not be too late to study a second language, and also the eclectic view, that the foreign language education should not be too early, either. Some educators and policy makers worry that an early foreign language education would interfere the natural development of children’s first language, then the notion of a Bilingual Kindergarten becomes controversial. The negative opinions are sometimes related to cultural conservativism which excludes foreign culture including language. After all, the basic idea of Critical Period Hypothesis confirms the notion that “the younger, the better”. 

2.2 Bilingual Advantage

Are bilingual people smarter? The reply to this question can be paradoxical and dialectical over time. Back to early 20th century, being bilingual was widely regarded as disadvantageous rather than an advantage. But a great shift came in the 1960s, when scientists began to know the benefits brought by a bilingual brain. As the technologies develop, such as brain imaging and psychological behavior experiments, new methods make it possible to put the study on a more precise and more empirical base. Researchers have found empirical evidences to support the hypotheses of bilingual advantage by experiments such as Dimensional Change Card Sorting, Ambiguous Figures Task, Frog Matrices Task, Flanker Task, Simon Task etc. They found that bilingual children have better executive function than monolingual peers, which includes three core elements: inhibitory control, cognitive flexibility and working memory. (Li Chuanjiang &Zhou Jin, 2015)