This dissertation examines how age affects the ability to produce intelligibility- enhancing speaking style adaptations in response to environment-related difficulties (noise-adapted speech) and in response to listeners’ perceptual difficulties (clear speech). Materials consisted of conversational and clear speech sentences produced in quiet and in response to noise by children (11-13 years), young adults (18-29 years), and older adults (60-84 years). Acoustic measures of global, segmental, and voice characteristics were obtained. Young adult listeners participated in word-recognition-in-noise and perceived age tasks. The study also examined relative talker intelligibility as well as the relationship between the acoustic measurements and intelligibility results. Several age-related differences in speaking style adaptation strategies were found. Children increased mean F0 and F1 more than adults in response to noise, and exhibited greater changes to voice quality when producing clear speech (increased HNR, decreased shimmer). Older adults lengthened pause duration more in clear speech compared to younger talkers. Word recognition in noise results revealed no age-related differences in the intelligibility of conversational speech. Noise-adapted and clear speech modifications increased intelligibility for all talker groups. However, the acoustic changes implemented by children when producing noise-adapted and clear speech were less efficient in enhancing intelligibility compared to the young adult talkers. Children were also less intelligible than older adults for speech produced in quiet. Results confirmed that the talkers formed 3 perceptually-distinct age groups. Correlation analyses revealed that relative talker intelligibility was consistent for conversational and clear speech in quiet. However, relative talker intelligibility was found to be more variable with the inclusion of additional speaking style adaptations. 1-3 kHz energy, speaking rate, vowel and pause durations all emerged as significant acoustic-phonetic predictors of intelligibility. This is the first study to investigate how clear speech and noise-adapted speech benefits interact with each other across multiple talker groups. The findings enhance our understanding of intelligibility variation across the lifespan and have implications for a number of applied realms, from audiologic rehabilitation to speech synthesis. / text