Native speakers of Japanese were shown single-character Kanji words which they read aloud as quickly as possible. Simultaneous multiple regression was used to investigate which of a set of nine variables predicted reading speed. Two factors exerted strong independent effects upon reading speed - the age at which the words enter a Japanese child's spoken vocabulary and the age at which Japanese children learn the characters that are used to write those words. Of three measures of word frequency, only rated familiarity made a significant independent contribution. There were no significant effects of word imageability, the number of alternative pronunciations of a Kanji character, the visual complexity of a character, or the length of the spoken word. We suggest that the quality of lexical representations is affected by the age at which they are learned, and that the effects of age of spoken and written acquisition reflect the influence of those variables upon the speech output lexicon and the visual input lexicon respectively.