All Soviet countries fell into sharp demographic/fertility collapse in the 90s when the USSR collapsed, and most never entirely pulled out of it, including Ukraine. So most have severe demographic issues in the ~20-30 year old range. Conscripting this group is reckless for these nations, because not only are there not many of them, but they're who should be creating the next generations. Here is Ukraine's population pyramid [1]. For contrast this is what an extremely healthy population pyramid looks like. [2]

[1] - https://en.wikipedia.org/wiki/File:Ukraine_population_pyrami...

[2] - https://en.wikipedia.org/wiki/Demographics_of_Israel#/media/...