Purpose: Atypical vowel production contributes to reduced speech intelligibility in children and adults with Down syndrome (DS). This study compares the acoustic data of the corner vowels /i/, /u/, /ae/, and /[open back unrounded vowel]/ from speakers with DS against typically developing/developed (TD) speakers. Method: Measurements of the fundamental frequency (f[subscript o]) and first four formant frequencies (F1-F4) were obtained from single word recordings containing the target vowels from 81 participants with DS (ages 3-54 years) and 293 TD speakers (ages 4-92 years), all native speakers of English. The data were used to construct developmental trajectories and to determine interspeaker and intraspeaker variability. Results: Trajectories for DS differed from TD based on age and sex, but the groups were similar with the striking change in f[subscript o] and F1-F4 frequencies around age 10 years. Findings confirm higher f[subscript o] in DS, and vowel-specific differences between DS and TD in F1 and F2 frequencies, but not F3 and F4. The measure of F2 differences of front-versus-back vowels was more sensitive of compression than reduced vowel space area/centralization across age and sex. Low vowels had more pronounced F2 compression as related to reduced speech intelligibility. Intraspeaker variability was significantly greater for DS than TD for nearly all frequency values across age. Discussion: Vowel production differences between DS and TD are age- and sex-specific, which helps explain contradictory results in previous studies. Increased intraspeaker variability across age in DS confirms the presence of a persisting motor speech disorder. Atypical vowel production in DS is common and related to dysmorphology, delayed development, and disordered motor control.