On the basis of their review of studies, Mayberry and Kluender (2017) propose that the human language learning ability becomes severely compromised if it is not developed in tandem with brain development in early childhood, but that it functions more or less flawlessly, even in adulthood, if language acquisition had at one time proceeded according to the maturational timetable. Mayberry and Kluender therefore suggest that the critical period hypothesis (CPH) for language is unambiguously tied to the timing of L1 acquisition, but that its relevance to L2 acquisition is less clear, the implication being that the well-documented AoA effects in the SLA literature are due to non-maturational (i.e., psychological, experiential, cross-linguistic, etc.) causes.
