This study examines vowel reduction as a function of stress and word length in 3 male and 3 female 3-, 5- and 7-year-olds, and adult controls. Participants produced triplets of two- and three-syllable words, where each Greek vowel /i, ε, ɐ, ο, u/ was studied in the first syllable in both stressed and unstressed conditions. Measurements were made for absolute and relative duration and (non-)normalized vowel space areas. The results showed that children had longer absolute and relative vowel duration and larger non-normalized vowel space areas than adults. Vowel space areas were lower and to the left in the acoustic vowel space in relation to adults. Longer duration and larger vowel space areas were found in stressed position across all ages with an adult-like spectral reduction already present at 3 years of age but an adult-like temporal reduction from 5 years of age and onwards for unstressed vowels in both two- and three-syllable words.