There have been 240 males and 18 females who have completed 20 or more freestyle Birkies since 1999. The individual results of these racers are provided in the graph above along with summary curves for each gender.
With these numbers, a longitudinal approach seems appropriate. The graph shows how variable individual performances can be, but it is still possible to provide a reasonable summary for each gender. These 20+ year Birkie skiers are accomplished and dedicated as shown by the prolonged and stable, if not improving, finish times from age 18 to the late 50s. The summary curve is very similar to the cross-sectional freestyle pattern of Birchleggers, and both emphasize the longevity of these skiers.
After 60, females show a relatively steep increase in finish times when compared to males, but patterns at the tails of these graphs, where there are few datapoints, are not as well-established as those in the middle years.
Note: Finish times are standardized to a 55k classical race or a 50k freestyle race.