Honestly, that's only implausible in a democratic society like the US. High command at a young age was standard practice for children of the aristocracy in many non-democratic societies. I can dig up examples if you like.
Indeed, and the country Selvaria serves is far from democratic. For reference, she is basically made a commander because she's superhuman, and having a walking WMD lead your team into battle works great for motivating the rank and file soldiers.
I spoke with an Army NCO the other day who helped train foreign officers at various point throughout his career. He said a Saudi he trained awhile back got a star or two on his uniform when he got home just for attending this US officer course. One of the coolest guys he trained, but completely incompetent. Guy was barely 20 or so.
OR you know this is anime logic like you have lolis and teenagers that fight with their lives and have a high position i have seen/read this all the time typical
