I still don't get it.

Usually you only go up the age-brackets when you are more and more desperate, for, you know, obvious reasons. Ukraine is doing it in reverse.

Ukraine is desperate. It's (currently) a war of attrition, it keeps going until one side suffers too much and can't replace stuff. Each side has constantly shifting limiting factors.

This is also why we've seen multiple headlines about Russia running out of stuff: they did run out of some things, then shifted to other stuff and ran out of that, then shifted to other stuff… — Ukraine has a shortage of both guns and young people right now.