This is a remarkable graph. There are enough 20+ yr freestyle Birchleggers (240 males and 18 females) to conduct a reasonable segmented regression analysis, and it shows, once again, the longevity, fitness, and endurance of those who continue to do the Birkie year after year for 20+ years, especially females. Female Birchleggers, on average, actually speed up until they reach 55 when they ski at about the same pace as males. After age 68, the genders diverge, and as seen in other graphs, females slow down faster than males.
This graph is very similar to the longitudinal graph summarizing Birkie Birchleggings freestyle result.
(More on statistical significance for estimates of breaklpoints and slopes.)
Note: Finish times are standardized to a 55k classical race or a 50k freestyle race.