Twenty-eight years of vowels: Tracking phonetic variation through young to middle age adulthood

•Individual speakers’ vowel spaces are tracked in a longitudinal corpus.•Generalized Additive Models (GAMs) reveal independent effects of talker age and vowel duration.•Vowel space size increase with talker age from ages 21 through 49.•The results based on automatic formant measurements were highly similar in the full corpus as in a hand-checked subset.•Implications for research on phonetic reduction and strengthening are discussed.