Answer
Because I was also a teenager once and I grew up reading and watching stories about child soldiers (i.e. "Gundam Wing", "Now and Then, Here and There") and continue enjoying those kinds of stories to this day (such as "Iron Blooded Orphens" and "Boku no Hero Academia"). Adult characters and teenage characters have a lot of different problems and they're both really fun to explore. Sometimes things just work out to be more interesting when the cast are teens, or vice versa. It entirely matters on just what I want to write. And get this, it isn't real. No real beings are harmed no matter what I do, so it's a great way to play around with all sorts of things!