What does a critical period for second language acquisition mean?: Reflections on Hartshorne et al. (2018).

Hartshorne et al. (2018) used a very large sample in order to disentangle the effects of age, years of experience, and age of exposure from each other in context of second-language acquisition. Participants were administered an online test of English grammar. Results revealed a critical period ending around 17 years of age for the most effective acquisition of a second language (L2). The findings of a late cutoff indicate the age range of late childhood to late adolescence as crucial for learning an L2. In this piece, we argue that these results can be conceptualized by emergentist models of language acquisition in which both behavior and brain interactively reorganize across development.