> As you said Ukraine’s demographic situation was quite horrible before the war. Very few people in their 20s

This is a perfect situation for waging war. Young people are prone to rebellion and overthrow the authorities that send them to war.

> Hence the conscription age being 27 earlier in the war. > seemed to be very politically unwilling to send actual conscripts there

It is exactly because of that reason. The younger the people, the more dangerous they are for the government.