Previous studies show that bilingual toddlers who develop their first language (L1) alongside another language can show early stabilization in the L1. This study investigates grammatical development of L1 Cantonese in children with very early onset of English before age 3 (earlier-onset bilinguals/EB, n = 31), with matched later-onset bilinguals (LB, n = 21) as the baseline. Input characteristics and child development measures at 3;0 and 5;8 were derived from parental reports, caretaker–child toy play and narration tasks. Results show that at 3;0, when the LB children were monolingual, the EB children were below the LB group in general grammatical complexity and seven specific grammatical structures (‘early costs’). At 5;8, the EB children converged with the LB children across grammatical measures in Cantonese, while demonstrating superior performance in English (‘long-term gains’). Our findings reveal a distinctive velocity of L1 development in early additive bilinguals raised in a bilingual society.
