Neural variability, or variation in brain signals, facilitates dynamic brain responses to ongoing demands. This flexibility is important during development from childhood to young adulthood, a period characterized by rapid changes in experience. However, little is known about how variability in moment-to-moment brain state engagement changes during development. Such investigations would require the continuous assessment of multiple brain states concurrently. Here, we leverage a new computational framework to characterize the state engagement variability (SEV) developmental trajectory. A consistent pattern of SEV changing with age is identified across cross-sectional and longitudinal datasets ( N > 3,000). The SEV developmental trajectory stabilizes around mid-adolescence, with timing varying by sex and brain state. SEV successfully predicts executive function (EF) in youth from an independent dataset. Deviations in SEV development are further linked to worse EF. These converging findings suggest that SEV changes over development, allowing individuals to flexibly recruit various brain states to meet evolving needs.