This study examines when the vocabulary knowledge of Japanese heritage speakers (HSs; N = 427, Mage = 9.96, female = 213) begins to diverge from monolingual counterparts (N = 136, Mage = 6.69, female = 65) and what factors explain individual differences in HS development. Vocabulary of HSs began to diverge from 5.61 years and this difference lasted until they were young adults. We also administered a fit‐for‐purpose questionnaire in 2021–2023 and identified six experiential latent factors: Holiday, School, Community, Proficiency, Literacy, and Home. Structural modeling indicates that Holiday predicted vocabulary scores, while Holiday and Literacy predicted Proficiency. Our findings highlight the importance of immersion experiences and literacy engagement for heritage language development.