Maintaining children's heritage language (HL) is a challenging task for linguistic minorities around the world. While many Japanese heritage children in Sydney attend weekend HL schools, they typically discontinue attendance before, or during, secondary school. To date, no longitudinal study has investigated what happens to their HL maintenance thereafter, taking into account family language policy factors and HL proficiency of subgroups with/without a "HL community of practice" membership. This article addresses this gap, analyzing the data from surveys and interviews with parents and youths (aged 17-21) whose HL practice and literacy were studied 10 years ago (Oriyama, 2012). In particular, I examine: (a) how HL communities of practice affect HL practice and proficiency; (b) how language ideology, practice, and proficiency correlate; and (c) how HL communities of practice affect language ideology. The between-group differences identified in language ideology, practice, and proficiency have implications for HL transmission and education.