Natural reading development gradually builds up to the adult vocabulary over a period of years. This has an effect on lexical processing: early-acquired words are processed more quickly and more accurately than later-acquired words. We present a connectionist model of reading, learning to map orthography onto phonology to simulate this natural reading development. The model learned early words more robustly than late words, and also showed interactions between age of acquisition and spelling-sound consistency that have been reported for skilled adult readers. In additional simulations, we demonstrated that age of acquisition effects are a consequence of incremental exposure to words in concert with changes in plasticity as learning proceeds, and are not due to uncontrolled differences in ease of reading between early and late-acquired words. Models which do not learn through cumulative training are unable to explain age of acquisition and related effects. (Contains 5 figures and 5 tables.)