Understanding pedestrian dynamics for different age compositions is paramount for enhancing their safety level on stairways in normal and evacuation conditions. In this study, we illustrate the findings of a series of single-file pedestrian flow experiments on stairways. This experiment was carried out with 131 participants having different age compositions, including preschool students (4–5 years), primary school students (8–10 years), middle school students (12–14 years), middle-aged pedestrians (27–57 years), and the elderly pedestrians (63–81 years). We collect data using three experimental settings: individual experiments with age homogeneity, single-file pedestrian experiments with age homogeneity, and single-file pedestrian experiments with age-mixed. Our findings show that age plays a substantive non-linear impact on individual unconstrained ascending and descending speeds. In addition, we developed a linear regression model that predicts the pedestrians’ free speed of different ages on stairways. Then, the effects of age compositions on pedestrian dynamics with fundamental diagrams (i.e., speed-density and specific flow-density relations) are analysed, and the maximum specific flow of age homogeneity is higher than that of age-mixed scenarios. Finally, we illustrate how the headway distance impacts participants’ ascending and descending speeds in both age-homogeneity and age-mixed conditions. As such, these new experimental results can provide the foundation for future calibration and validation of pedestrian evacuation models as well as insight into safety guidelines.