Age of acquisition (AoA) refers to the age at which people learn a particular item and the AoA effect refers to the phenomenon that early-acquired items are processed more quickly and accurately than those acquired later. Over several decades, the AoA effect has been investigated using neuroscientific, behavioral, corpus and computational techniques. We review the current evidence for the AoA effect stemming from a range of methodologies and paradigms and apply these findings to current explanations of how and where the AoA effect occurs. We conclude that the AoA effect can be found both in the connections between levels of representations and within these representations themselves, and that the effect itself occurs through the process of the distinct coding of early and late items, together with the nature of the connections between levels of representation. This approach strongly suggests that the AoA effect results from the construction of perceptual-semantic representations and the mappings between representations.