The purpose of this longitudinal study was to evaluate the influence of age (adult vs. child) and length of residence (LOR) in an L2-speaking country (3 vs. 5 years) on degree of foreign accent in a second language (L2). Korean adults and children living in North America, and age-matched groups of native English (NE) adults and children, recorded English sentences in sessions held 1.2 years apart (T1 vs. T2). NE-speaking listeners rated the sentences for overall degree of perceived foreign accent using a 9-point scale. The native Korean (NK) children received significantly higher ratings than the NK adults did, but lower ratings than the NE children. The NK children—even those who had arrived as young children and been enrolled in English-medium schools for an average of 4 years—spoke English with detectable foreign accents. The effects of LOR and the T1–T2 differences were non-significant for both the NK adults and the NK children. The findings were inconsistent with the hypothesis that adult–child differences in L2 speech learning are due to the passing of a critical period. The suggestion is made that the milder foreign accents observed for children than adults are due, at least in part, to the greater L2 input typically received by immigrant children than adults.