The reason the trend line drops over 60 is that there are fewer people over 60 taking the test and thus, the questionable responses have a bigger influence. If you sample less from that cohort, it will be less likely that your sample is representative.
Yes, while I like the article, it kind of distorts the evaluation in the original paper, where they don't only show the trend line but also it's 95% confidence interval (which very obviously significantly widens with age).
