DownwardDog
Riding a Brompton with a power meter.
The relevant section starts at 25:45.
A couple of factors are at play
First, both Ukraine and Russia have similar demographic pyramids, with a steep decline in births following the collapse of the USSR.
View attachment 403020
Ruling out retirees and children, the average age of the "fighting" population is already 41.
Second, the UKR army has exemptions from mobilisation for people without military experience between the ages of 18 and 27.
This means that the army's pre-war cohort of young contract soldiers has not been replenished with conscripts of similar age, pushing the average higher.
That 44m is a 2020 number. Since then 2m Ukrainians have gone to Russia, 4m went to the EU+UK and Ukraine have carelessly misplaced the best part of four oblasts. So the Ukrainian population under the control of Kiev is probably closer to 30m with horrendous demographics.