Explaining Age-Related Effects in Bilingual Language Acquisition: The Interplay of Neurodevelopment, Language Exposure, and Learning Mechanisms

For decades, the age-related disparity in second language learning outcomes has remained one of the most debated core topics in bilingual acquisition research, with long-standing disagreements over whether observed differences stem from immutable biological critical periods, variable environmental input, or distinct cognitive learning patterns across life stages. This literature review examines how age shapes differences in bilingual language acquisition between child and adult learners by considering three dimensions: brain development, learning environment and cognitive processes. Research on neurodevelopment suggests that children and adults both have different linguistic advantages in phonological and grammatical development. Studies of language exposure indicate that quantity, quality, and richness of input significantly shape bilingual development. Research on cognitive and learning processes further suggests that children and adults may both acquire a second language under implicit and explicit conditions. Drawing on existing literature in child and adult linguistic studies, the literature argues that age-related differences in bilingual acquisition are not completely determined by biological development but emerge from interactions among neurodevelopmental plasticity, environmental exposure and learning awareness.