Four experiments investigated infants' preferences for age-appropriate and age-inappropriate infant-directed speech (IDS) over adult-directed speech (ADS). Two initial experiments showed that 6-, 10-, and 14-month-olds preferred IDS directed toward younger infants, and 4-, 8-, 10-, and 14-month-olds, but not 6-month-olds, preferred IDS directed toward older infants. In Experiment 3, 6-month-olds preferred IDS directed toward older infants when the frequency of repeated utterances matched IDS to younger infants. In Experiment 4, 6-month-olds preferred repeated IDS utterances over the same IDS utterances organized without repetition. Attention to repeated utterances precedes word segmentation and sensitivity to statistical cues in continuous speech, and might play a role in the discovery of these and other aspects of linguistic structure. (Contains 2 figures, 4 tables and 2 footnotes.)