["Psychophysiology, Volume 63, Issue 5, May 2026. ", "\nABSTRACT\nExtant literature suggests that developmental improvements in processing speed reflect changes in a common global processing factor. In theory, then, the influence of age on processing speed should be shared across premotor processes (e.g., response selection) and motor processes (e.g., response execution). However, some researchers have observed differences in the effect of age on speed across different processes depending on stage of development, and research on neurodevelopment has long demonstrated variation in the developmental trajectory of cortical regions associated with different functions. The current study explored whether age‐related differences in processing speed during adolescence varied between premotor and motor domains, testing whether these domain‐specific differences accounted for age‐related variance in choice reaction time (RT). Adolescent participants (N = 204, 68.6% female) varying in age from 14 to 19 years (Mage = 16, SDage = 1.73) completed a flanker task while EEG was recorded. We quantified the lateralized readiness potential (LRP) to fractionate RTs into premotor (stimulus‐locked LRP [S‐LRP]) and motor (response‐locked LRP [R‐LRP]) intervals. Both S‐LRP and R‐LRP latencies correlated with RT, but only S‐LRP latency decreased with age. Mediation analysis confirmed a significant indirect effect of age on RT through S‐LRP latency but not R‐LRP latency, suggesting that faster processing speed among older adolescents stems from faster premotor—but not motor—processing. We demonstrate the utility of using LRP latencies to investigate domain‐specific processing speed, highlighting directions for future work to link structural development research to functional measurements.\n"]