Patkowski's first question, `Will there be a difference between learners who began to learn English before puberty and those who began learning English later?, was answered with a very resounding `yes'. When he examined the other factors which might be thought to affect success in second language acquisition, the picture was much less clear. There was, naturally, some relationship between these other factors and learning success. However, it often turned out that age was so closely related to the other factors that it was not really possible to separate them completely. For example, length of residence in the United States sometimes seemed to be a fairly good predictor. However, while it was true that a person who had lived in the country for fifteen years might speak better than one who had been there for only ten years, it was often the case that the one with longer residence had also arrived at an earlier age. However, a person who had arrived in the United States at the age of eighteen and had lived there for twenty years did not score significantly better than someone who had arrived at the age of eighteen but had only lived there for ten years. Similarly, amount of instruction, when separated from age, did not predict success to the extent that age of immigration did.
Thus, Patkowski found that age of acquisition is a very important factor in setting limits on the development of native-like mastery of a second language and that this limitation does not apply only to accent. These results gave added support to the critical period hypothesis for second language acquisition.

Figure 3. I: Bar charts showing the language levels ofpre- and post puberty learners ofEnglish (Patkowski 1980).
Experience and research have shown that native-like mastery of the spoken language is difficult to attain by older learners. Surprisingly, even the ability to distinguish between grammatical and ungrammatical sentences in a second language appears to be affected by the age factor, as we will see in the next study by Newport and Johnson.
Intuitions of grammaticality
Jacqueline Johnson and Elissa Newport conducted a study of 46 Chinese and Korean speakers who had begun to learn English at different ages. All subjects were students or faculty at an American university and all had been in the United States for at least three years. The study also included 23 native speakers of English (Johnson and Newport 1989).

The participants in the study were given a grammaticality judgement task which tested twelve rules of English morphology and syntax (verb tense, noun pluralization, verb agreement, word order, question formation, use of articles, and use of pronouns). They heard sentences on a tape and had to indicate whether or not each sentence was correct. Half of the sentences were grammatical, half were not.
When they scored the tests, Johnson and Newport found that age of arrival was a significant predictor of success on the test. When they grouped the learners in the same way as Patkowski, comparing those who began their intensive exposure to English between the ages of three and fifteen with those who arrived in the United States between the ages of seventeen and 39, once again they found that there was a strong relationship between an early start to language learning and performance in the second language. Johnson and Newport noted that before the age of fifteen, and especially before the age of ten, there are few individual differences in second language ability. Older learners will not have native-like language skills and are more likely to differ greatly from one another in ultimate attainment.
This study, then, further supports the hypothesis that there is a critical period for attaining full native-like mastery of a second language. Nevertheless, there is some research which suggests that older learners may have an advantage, at least in the early stages of second language learning.