One frequently cited factor is the age of the learner, with the assumption that younger learners acquire a second language more quickly and with a higher level of proficiency. Periodic reviews of this literature (Bialystok and Hakuta, 1994; Collier, 1987; Epstein et al., 1996; Harley and Wang, in press; Krashen et al., 1982; Long, 1990; Snow, 1987) have not supported this claim very well. Even though there is a critical period in the learning of a first language, this does not imply there is one for second-language learning. The following observations might be made:•More mature learners generally make faster initial progress in acquiring morphological, syntactic, and lexical aspects of a second language.•An increasing age of onset for second-language acquisition is correlated with declining ultimate attainment in the control of phonological, morphological, and syntactic aspects of language across age groups, beginning typically by age 6-7 in childhood and continuing into adulthood. In adult learners, this association between onset age and declining outcomes is most strongly manifested in oral aspects of second-language proficiency (maintenance of an accent).•Some adult learners are nonetheless capable of near-native, if not native-like, performance in a second language, while some children are unsuccessful in achieving native-like performance.•There is a general lack of evidence that acquisition processes differ across age groups, i.e., that radically different types of errors are made or that there is a different sequence to the acquisition of structures for learners of different ages (Harley and Wang, in press).