Considering Holly’s posting at the start of the comic, that is, her being seconded to a military unit, albeit one that was on a ‘low-stakes’ assignment, sort of implies that this is the case, or that the ‘age of majority’ is lower.

We also have to remember that this is a society that has been running with Magical Girls for centuries. A world where, IIRC, The First Generation acted in secret, and by the time the Next Generation cropped up, they had the social clout to make sure that the kiddles would be okay.

so much of being a Magical Girl in this setting is tied into the tropes being reality(Strong emotion bringing out new levels of power, that power frequently being activated in a do-or-die crisis, et cetera) that the actual governmental policy may be to, so long as they are acting in the common good, just let the Tykebomb keep on keeping on. That when they transgress the law, the law was wrong, if only in this situation, and to, when possible, aim them in the direction of Most Societal Good.

TL;DR Ivy is almost certainly not violating any child labor laws now, but there may have been some constitutional amendments to allow Ceannis to field the Humanoid Anti-Typhoon for the benefit of the continent.