Previous work suggests that adolescents are still refining acoustic-phonetic cue use in clear-speech perception. This study shows adolescents’ immature perception of reduced speech, in which speech sounds are naturally deleted and merged within and across words. German adults and 16-year-olds listened to either German reduced or unreduced (few or full cues) part- and full phrases (without and with context) in a phrase-intelligibility task. As expected, adolescents had lower scores when adequate perception required flexible acoustic-phonetic cue use most, i.e., when hearing reduced speech without context. Participants also listened to reduced and unreduced words and pseudowords (no context) in a lexical decision task. Here, 16-year-olds had poorer and slower responses than adults overall and particularly when hearing pseudowords. Explanations for the age effects are discussed. We conclude that experience continues to refine linguistic representations, at least until adulthood.