This study evaluated whether age effects on second language (L2) speech learning derive from changes in how the native language (L1) and L2 sound systems interact. According to the interaction hypothesisg (IH), the older the L2 learner, the less likely the learner is able to establish new vowel categories needed for accurate L2 vowel production and perception because, with age, L1 vowel categories become more likely to perceptually encompass neighboring L2 vowels. These IH predictions were evaluated in two experiments involving 64 native Korean- and English-speaking children and adults. Experiment 1 determined, as predicted, that the Korean children were less likely than the Korean adults to perceive L2 vowels as instances of a single L1 vowel category. Experiment 2 showed that the Korean children surpassed the Korean adults in production of certain vowels but equaled them in vowel perception. These findings, which partially support the IH, are discussed in relation to L2 speech learning.