What can be the causes of the difference in performance?
Are those reliable differences? Without knowing what confidence intervals you plotted it's difficult to estimate visually (95%? ±1 SD?), but I wouldn't be surprised that there are no statistically significant performance differences between those levels. Keep in mind that if this test emerged only after seeing the data, rather than a priori based on theory, then that changes how to interpret the significance of such tests and makes them less trustworthy:


  HARKing: Hypothesizing After the Results are Known http://journals.sagepub.com/doi/abs/10.1207/s15327957pspr0203_4


The main finding seems to be an increase with age, possibly non-linear. Thresholding could be due to the task complexity.