Age of Acquisition
Age of acquisition (AoA), also called age of onset (AoO), is the age at which a learner begins to receive substantial input in a second language. It is one of the most studied and most misunderstood variables in SLA: easy to measure, strongly correlated with outcomes in naturalistic settings, far less straightforward in instructed foreign-language classrooms.
The Naturalistic Finding
In immigrant and long-residence naturalistic contexts, AoA shows a strong and consistent negative correlation with nativelike ultimate attainment. Later starters reach lower plateaus, with the clearest break around puberty on phonology and a more gradual decline on morphosyntax. This pattern is the empirical ground for the Critical Period Hypothesis and for Bley-Vroman's Fundamental Difference Hypothesis.
The Stockholm programme of Kenneth Hyltenstam, Niclas Abrahamsson, and Bylund has sharpened the finding by distinguishing near-native from nativelike. When late starters who pass listener judgement are scrutinised on sensitive grammatical and phonetic tasks, virtually none reach the native range, even at ages of onset in late childhood.
The Instructed Foreign-Language Finding
Inside the classroom, the naturalistic story does not transfer cleanly. Carmen Muñoz's Barcelona Age Factor (BAF) Project, controlling for hours of exposure, found that older starters outperform younger starters on rate of acquisition at equivalent input. "Earlier is better" holds for naturalistic immersion, not for a few hours a week of foreign-language classes in a school system. The policy move to push English instruction into earlier grades is, on Muñoz's evidence, poorly justified.
What It Does Not Mean
Two common misreadings need resisting:
- AoA is not the same thing as the critical period. AoA is a variable; CPH is a theoretical claim about why AoA matters. The variable can show strong effects even if CPH is wrong.
- AoA effects are not decisive proof of maturational constraints. Length of residence, quality of input, use of L1, and aptitude all co-vary with AoA, and disentangling them requires careful design.
Measurement Issues
AoA sounds simple but is harder than it looks. Decisions about what counts as "beginning to acquire" — first exposure? first sustained input? arrival in the host country? — produce different numbers for the same learner. Studies that find different AoA effects sometimes disagree because they have operationalised the variable differently.
References
- Muñoz, C. (Ed.). (2006). Age and the Rate of Foreign Language Learning. Multilingual Matters.
- Abrahamsson, N. & Hyltenstam, K. (2009). Age of onset and nativelikeness in a second language. Language Learning, 59(2), 249–306.
- Bylund, E., Abrahamsson, N. & Hyltenstam, K. (2012). Age of acquisition effects or effects of bilingualism in L2 ultimate attainment? In Granena & Long (Eds.), Sensitive Periods, Language Aptitude, and Ultimate L2 Attainment. John Benjamins.
- Birdsong, D. (2006). Age and second language acquisition and processing: A selective overview. Language Learning, 56(s1), 9–49.