Research on age-related changes in speech has primarily focused on comparing “young” vs. “elderly” adults. Yet, listeners are able to guess talker age more accurately than a binary distinction would imply, suggesting that acoustic characteristics of speech change continually and gradually throughout adulthood. We describe acoustic properties of vowels produced by eleven talkers based on naturalistic speech samples spanning a period of 28 years, from ages 21 to 49. We find that the position of vowels in F1/F2 space shifts towards the periphery with increasing talker age. Based on Generalized Additive Mixed Effects models, we show that this shift is not fully attributable to changes in vowel duration or to segmental context. We discuss the implications of our results for research on aging and speech, and for research in which durational shortening and spectral characteristics of vowels are assumed to reflect a unitary process of phonetic reduction.