What are minors even doing with military? System must be a bit curious to allow 15yo.
Military schools for intermediate and high school grades.
An old Royal Naval tradition that Australia inherited - boys were taken in for training as officers at 13 until the late 1950s. The age was raised to 16 until the late 1970s - I started at 16 myself in 1972 (I experienced and saw no sign of this abuse, although I do not doubt there were cases where it happened, I believe it was very rare). There were also Junior Recruits who were taken for training at 15 until quite recently - they trained to be sailors, not officers.
Even today you can join at 17, although you will not be deployed overseas until you are 18.
Why is this way? Partly it was tradition, but it also reflected our school systems in Australia. It used to be normal here - until the 1980s - for students to finish school at 15 and go to work. You only stayed on for the last two years of secondary school if there was a specific reason you needed to - most commonly because you were aiming at a white colour job or planned to go to university. And even now, a lot of students will still be 17 when they finish secondary schooling - kids on a normal education track start turning 18 in the second half of their final school year - if they couldn't move on to their future plans until they turned 18, we'd have kids who finished school and wanted to join the Defence Forces who would have potentially have to wait six months or more for their eighteenth birthday.