This study investigated age-related differences in working memory and inhibitory control (WMIC) in 3 1/2-, 4-, and 4 1/2-year-olds and how these differences were associated with differences in regulatory aspects of temperament, language comprehension, and brain electrical activity. A series of cognitive control tasks was administered to measure WMIC ability, including the Stroop-like day-night and the yes-no tasks. Baseline and task electroencephalographic data were collected. The Children's Behavior Questionnaire was used to assess caregiver perceptions of temperament with a particular interest in the effortful control and surgency factors, and language comprehension was measured with the Peabody-Picture Vocabulary Test-III. The results of this study demonstrated differential temperament-cognition relations for the three age groups, as positive associations were found between WMIC and effortful control for the 3 1/2- and 4-year-olds and negative associations were found between WMIC and surgency for the 4 1/2-year-olds. An increasingly robust relation between WMIC and language comprehension was demonstrated across the three age groups, as well as differential patterns of task-related brain electrical activity.