To arrive at this graph, the five fastest skiers were identified for each age in each year from 1999 to 2025. There have been 22 “full length” point-to-point Birkies since 1999, so each data point in the graph represents the average of 110 top 5 results for each gender.
Both genders have three transition ages, and the rate of change is roughly similar during the periods between transition ages. During the first period from 18 to 25, both genders increase their speed. Thereafter, skiers of both genders show a similar pattern of slowdown, although males transition 6 to 11 years later than females.
After age 76, males slow down by 22:36 minutes:sec/yr, and after age 65, females slow by 14:36 minutes:sec/yr.
(More on statistical significance for estimates of breakpoints and slopes.)
Note: Finish times are standardized to a 55k classical race or a 50k freestyle race.