What can be the causes of the difference in performance? The box-plot above represents the number of programming tasks performed correctly in 30 minutes during an experiment by me. Each box represents an age group:


p1 = 6 - 7 years old
p2 = 7 - 8 years old
...
p6 = 11 - 12 years old
s1 = 12 - 13 years old
s2 = 13 - 14 years old
...
b1 = 16 - 17 years old
b2 = 17 - 18 years old


When I see the results, I am surprised by the difference between p4, p5 and p6.
The set of tasks was almost identical by age. The only difference is that by increasing the age they have some less start tasks.
I think it may be due to the ability of reading comprehension and motor development ...

Are there more factors that can influence?

UPDATE
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.