Cognitive development is shaped by interactions between cognitive architecture and environmental experiences of the growing brain. We examined the extent to which this interaction during development could be observed in language processing. We focused on age of acquisition (AoA) effects in reading, where early-learned words tend to be processed more quickly and accurately relative to later-learned words. We implemented a computational model including representations of print, sound and meaning of words, with training based on children’s gradual exposure to language. The model produced AoA effects in reading and lexical decision, replicating the larger effects of AoA when semantic representations are involved. Further, the model predicted that AoA would relate to differing use of the reading system, with words acquired before versus after literacy onset with distinctive accessing of meaning and sound representations. An analysis of behaviour from the English Lexicon project was consistent with the predictions: Words acquired before literacy are more likely to access meaning via sound, showing a suppressed AoA effect, whereas words acquired after literacy rely more on direct print to meaning mappings, showing an exaggerated AoA effect. The reading system reveals vestigial traces of acquisition reflected in differing use of word representations during reading.