I love how you continue to dig into this. Keep up the great work. I do feel compelled to highlight a minor point that I am sure you know but did not mention. WIth respect to "Second, there’s a divergence in 2021" recall that there was a schedule change in 2021 that placed Boston's marathon the day after Chicago's. With all three of these races now scheduled over the course of 4 weeks, most runners (not me, however, because I ran all three as well as Berlin and London that year) were forced to choose which of them to run. Did that have an affect on the age distribution? I don't know but it is clearly a one-off outlier situation that doesn't demand further analysis.