Unpredictable, it certainly wasn't, much as any sane person might wish it hadn't been necessary.
Ukraine has had a clearly set out plan for phased mobilization for a number of years. You can read about it here: https://en.wikipedia.org/wiki/Mobilization_in_Ukraine
Ukraine has been understandably reluctant to enlist its younger generational cohort (along with those who have multiple children and other contraindicationg factors) to add to the toll on its demographics that Putin's aggression has brought about.
Much has been made by various media and Putin propagandists for the last year or so of the rising average age of Ukrainian fighters. Part of the reason for that is because the earlier phases of mobilization concentrated on those who were older and had military experience.
Beyond the lurid headlines, this is a matter of cold assessment and allocation of human resources. As Cornell University observed:
“On the rationalization side, the Ukrainians need to know who is out there, in terms of manpower. They need to know the ages and jobs of the men they might draft, so that they can make choices that will ensure the most efficient use of manpower. Drafting a man who maintains the Ukrainian railway system just causes that vital system problems while adding only one more (untrained) soldier to the military. Better to leave such an engineer there and draft someone in a less immediately critical job.
“Large industrial wars like this one are as much about organization as they are about fighting, and this is a sign that Ukraine takes that lesson seriously.”
https://as.cornell.edu/news/ukraines-mobilization-bill-sign-desperation-and-rationalizationAs a matter of perspective, the lower age for conscription in Ukraine is now 25, reduced from 27. In the Vietnam War, the lower limit was 18, though younger men did serve and die, and the average age of US troops was something over 22 (not 19, as the myth would have it), men were eligible for the draft until age 35, and the average age of combatants in the Second World War was 26.
More concerning is that the new moves counter the previous plans for rotation of troops after serving for certain periods at the front. That will obviously take a toll on morale and stamina.
It should not be ignored that the current phase of mobilization might not have been necessary without the stonewalling from the Trumpite wing of the Republicans.
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