Early acquired words are processed faster than later acquired words in lexical and semantic tasks. Demonstrating such age of acquisition (AoA) effects beyond reasonable doubt, and then investigating those effects empirically, is complicated by the natural correlation between AoA and other word properties such as frequency and imageability. In an effort to find a laboratory analog of AoA effects which would allow such issues to be addressed more easily, we conducted three experiments in which participants learned foreign words, with some (‘early’) words trained from the outset while other (‘late’) words were introduced some time later then interleaved with the early words. Order of acquisition effects were observed in picture naming, lexical decision and semantic categorization, persisting for several weeks after the end of training. The results demonstrate an important role for order of acquisition in the formation of lexical representations that is independent of other factors such as cumulative frequency, frequency trajectory and imageability. Analyses of cumulative learning effects offer the potential to investigate the differential impact of early and later experiences on the formation of lexical and other mental representations. The discovery of order of acquisition effects in word learning also has implications for classroom teaching of second language vocabulary.