Objective: The purpose of this research was to investigate the age-related changes in whole-body motor variability during sit-to-stand (STS) task. It has been reported that children perform motor tasks less accurately with greater variability as compared to adults. However, it is still unknown how they utilize the abundant degrees of freedom and accomplish voluntary actions. Uncontrolled manifold (UCM) analysis has been used to partition motor variabilities into two independent variability components, task-relevant variability (ORT ) and task-irrelevant variability (𝑉UCM). We investigated what differences exist between children and adults with respect to these two motor variability components in relation to motor development stages. Method: Ten 6-year-old children (height: 116.2 ± 4.3 cm, weight: 23.1 ± 3.9 kg, motor development assessment percentile score: 77.5 ± 18.6%), ten 10-year-old children (height: 138.7 ± 7.2 cm, weight: 35.8 ± 10.3 kg, motor development assessment percentile score: 73.9 ± 12.7%), and ten young adults (age: 23 ± 1.6 year-old, height: 164.3 ± 11.4 cm, weight: 60.8 ± 12.0 kg) participated in this study. Each participant performed STS ten times, and a motion capture system was used to capture the whole-body kinematics. Each segment centers of mass and the whole-body center of mass were calculated, and UCM analysis was used to quantify motor variabilities, 𝑉ORT and 𝑉UCM. One-way ANOVA was used for statistical analysis. Results: We found that children produced more motor variabilities in 𝑉ORT and 𝑉UVM in all three dimensions, anterior-posterior, medial-lateral, and vertical. As age increased, both, 𝑉ORT and 𝑉UCM significantly decreased (p<.05). Conclusion: The greater 𝑉ORT found in children compared to adults indicates that the repeatability over repetitions improves through development, while the greater 𝑉UCM found in children suggests that children better utilize the abundant degrees of freedom during STS compared to adults.