Age of learning affects the learning of L2 pronunciation but not morphosyntax

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Many studies have shown that performance in a second language (L2) decreases as the age of L2 learning increases, but such findings are difficult to interpret because many factors are confounded with age. Here we assessed the overall degree of foreign accent in English sentences, and scores obtained on a 144-item grammaticality judgment test (GJT). The subjects were 240 Koreans who differed according to their age of arrival (AOA) in the United States (2 to 23 years). As AOA increased, foreign accents grew stronger and the GJT scores decreased steadily. However, when variables confounded with AOA were controlled using a subgroup matching technique, AOA was found to exert a significant effect on the foreign accent ratings but not on the GJT scores. Total years of education in the United States, but not language use or AOA, significantly affected scores for a subset of GJT items that tested rule-based, generalizable aspects of English morphosyntax. Conversely, language use, but not education or AOA, affected the scores for GJT sentences testing lexically based aspects of morphosyntax. Age effects on L2 pronunciation thus appear to be real. However, differences in morphosyntactic knowledge may instead be due to age-related variation in education or English-language use.