Dystopian Child Soldier Mystery “Children’s Land” vol 2 by Kashimiharu.
Dystopian Action set in a country that is only inhabited by children under the age of 12 years fighting against terrifying creatures called ‘adults’ with their own army. In order to avoid turning into these dark monsters themselves, the kids have to sleep in cryogenic chambers once they are older than 12yo. But one day when the brother of a certain boy with powerful magic abilities turns into such an adult creature, he decides to join the children’s army in order to turn his brother back to his original state.
(Kodomo no Kuni)