What causes the effect of age-of-acquisition in lexical processing?

Three hypotheses for effects of age of acquisition (AoA) in lexical processing are compared: the cumulative frequency hypothesis (frequency and AoA both influence the number of encounters with a word, which influences processing speed), the semantic hypothesis (early-acquired words are processed faster because they are more central in the semantic network) and the neural network model (early-acquired words are faster because they are acquired when a network has maximum plasticity). In a regression study of lexical decision (LD) and semantic categorization (SC) in Italian and Dutch, contrary to the cumulative frequency hypothesis, AoA-coefficients were larger than frequency-coefficients and contrary to the semantic hypothesis, the effect of AoA was not larger in SC than in LD. The neural network model was supported.