This cross-sectional study investigated the bilingual lexical skills of 175 US school-age children (5 to 18 years old) with Cantonese, Mandarin, or Korean as their heritage language (HL), and English as their dominant language. Primary study goals were to identify potential patterns of development in bilingual lexical skills over the elementary to high school time span and to examine the relation of environmental factors to lexical skills. HL and English productive lexical skills were assessed with a Picture Naming and a Verbal Fluency task. English receptive lexical skills were assessed with Peabody Picture Vocabulary Test. A survey obtained information about participants' language use in six environmental contexts. There were age-related significant increases in both HL and English skills. However, English proficiency already had a significant lead over HL proficiency at the youngest age. English receptive lexical skills reached monolingual expectations from age 8, whereas for HL, high school age participants on average only reached the level of early elementary school monolinguals. Although more English use at home at younger ages was associated with stronger English skills, the relation did not exist for older participants. Instead, among older participants, more English use at home was associated with weaker HL skills. Children's attendance at HL programs and visits to home countries bore little relation to HL proficiency.