The critical period hypothesis for language learning2007

"A critical period for language learning is often defined as a sharp decline in learning outcomes with age. This study examines the relevance of the critical period to English proficiency among immigrants in the US. It uses microdata from the 2000 US Census, a model of language acquisition from the economics and sociology literatures, and a flexible specification of an estimating equation based on 64 age-at-migration dichotomous variables. It shows that self-reported English language speaking proficiency among immigrants declines more-or-less monotonically with age at migration, and this relationship is not characterized by any sharp decline or discontinuity that might be considered consistent with a "critical" period. The findings are robust across the various immigrant samples, and between the genders"--Forschungsinstitut zur Zukunft der Arbeit web site.