Stack Overflow reputation is cumulative. This means that if two people are providing answers of the same quality and at the same rate over time, the folks who have been there longest will have higher reputations, and that the higher reputation will reflect only tenure. Not any kind of quality.
If you want to look at quality, you should be looking at a metric that is something like (total reputation / number of months active). Even this is imperfect of course, since if people take a hiatus or something that will present the appearance of worse quality using this metric.
I was going to say that this fatal flaw invalidated the conclusions because the correlation between reputation and age just reflected the older people being around longer. The problem with that is that Stack Overflow opened in 2008. That's not enough time to explain a linear trend that tracks from age 16 to nearly age 50, but the final conclusion "So, senior coders earn their higher reputation by providing more answers, not by having answers of (significantly) higher quality." should still be re-examined with tenure-controlled analysis to try and see whether older aged members have been members longer.