Also, if you look at the right side of figure 1 there are lines on the bottom of the complexity chart for each of the tasks, presumably representing people who

Also, if you look at the right side of figure 1 there are lines on the bottom of the complexity chart for each of the tasks, presumably representing people who essentially did not do the task (although they still seem to have spent a variable amount of time not doing the task). It looks like this skews older, and just by eyeballing the chart some of the 75+ would be upward trends if not for that bottom line. I skimmed the article and did not see a discussion of this.