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The acquisition of new skills over a life span is a remarkable human ability. This ability, however, is constrained by age of acquisition (AoA); that is, the age at which learning occurs significantly affects the outcome. This is most clearly reflected in domains such as language, music, and athletics. This article provides a perspective on the neural and computational mechanisms underlying AoA in language acquisition. The authors show how AoA modulates both monolingual lexical processing and bilingual language acquisition. They consider the conditions under which syntactic processing and semantic processing may be differentially sensitive to AoA effects in second-language acquisition. The authors conclude that AoA effects are pervasive and that the neural and computational mechanisms underlying learning and sensorimotor integration provide a general account of these effects.

How does the brain manage to store and process multiple languages without encountering massive interference and transfer? Unless we believe that bilinguals live in two totally unconnected cognitive worlds, we would expect far more transfer than actually occurs. However, imaging and lesion studies have not provided consistent evidence for the strict neuronal separation predicted by the theory of modularity. We suggest that emergentist theory offers a promising alternative. It emphasizes the competitive interplay between multiple languages during childhood and by focusing on the dual action of competition and entrenchment, avoids the need to invoke a critical period to account for age of acquisition effects in second-language learning. This view instantiates the motto formulated by Elizabeth Bates that'modules are made, not born.'

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