According to many researchers Johnson and Newport (1989) have provided important support for the existence of the critical period of the second language acquisition. Their main evidence is concluded into two studies.
The first one is taken on 46 native Chinese and Korean learners of the English as second language, who were exposed to this language by native speakers in the US. Because of their wide age differentiation these learners were divided into two groups, early learners, who came in the US before their age of 15, and late arrivals, who came in the US before their age of 17. In order the research results to be valid, there were only two conditions for the examiners, they had to live constantly in the US for at least three years and be exposed to the English language for at least five years. The main aim of this study was to test whether has some close relationship between examiners’ age of learning English as a second language and their performance on the test of English morphology and syntax. It is clear that the results of this study are related with age differences, or in other words after the age of 7 some maturational factors restrict the second language acquisition. This means that the examiners that started acquiring the English language earlier than those who began learning the language later, shown better results, which justifies the critical period as a phenomenon.
The second research is taken on 2.3 million immigrants with Chinese and Spanish background on the age of 15-20 during a long period of time, in search for evidence of discontinuity in the English level in order the Critical Period Hypothesis to be disproved. Individuals were tested through form in which they used to self-describe their English ability using one of the five categories: “not at all”, “not well”, “well”, “very well”, and “speak only English”. The results showed a great success in acquiring the English as second language which was continuously declining through life span. From the taken studies, Johnson and Newport concluded that a critical period exists for a grammar acquisition and that the age 15 is the end point of the critical period for second language device.
For more detailed information about these researches implemented by Johnson and Newport visit the web page:Critical Period Effects in Second Language Learning by Johnson & Newport