This study examined the production and perception of English vowels by native Korean (NK) learners of English on two occasions separated by about 1 year. A preliminary experiment revealed that NK adults classified some pairs of contrastive English vowels using two different Korean vowels whereas other pairs showed classification overlap, implying they would be difficult for Korean learners of English to discriminate. In two subsequent experiments, NK adults and children differing in length of residence in North America (3 vs. 5 years; 4 groups of 18 each) were compared to age-matched native English (NE) speakers. In Experiment 2, NK children were found to discriminate English vowels more accurately than NK adults but less accurately than NE children. In Experiment 3, English words containing /i e ε æ / were elicited using a picture-naming task. Some vowels produced by NK children were heard as intended significantly more often than vowels produced by NK adults. Acoustic analyses revealed that NK children produced significantly larger between-vowel contrasts than NK adults but did not differ from NE children. Taken together, the results suggested that although children are more successful than adults in learning the phonetic properties of second-language vowels, they might continue to differ from age-matched native speakers in certain respects as uncovered by the vowel discrimination test in Experiment 2.