As a society we hold people responsible for their actions beyond a certain age. That particular age might vary from country to country, but everyone will agree

As a society we hold people responsible for their actions beyond a certain age. That particular age might vary from country to country, but everyone will agree that a 21 year old can face any consequence of a decision they've made. That's why they are allowed to drink, vote, join the military etc. Everyone will also agree that a 12 year old does not understand the consequences of their actions. Such a child might not know anything other than murdering people but also might not be capable of evaluating their own actions with respect to right vs wrong. This is also why there is an alternate legal system for minors accused of crimes. Now a peacekeeper who shoots an armed child has to live with the knowledge that they probably killed someone who didn't know any better. They've been the judge, jury and executioner in a trial that lasted a split-second, despite the fact that children get a lot of leeway when it comes to most crimes.