FFS people.
First and foremost, this is a mostly dark fun gag manga. It's fictional, they've got magical powers, governments for some reason aren't going insane over it and posting military everywhere, etc. That's the context, it's not the real world. A lot of the debates we have change when the "kids" are superhuman armies-of-one who could annihilate any of us. And flat-out this series just doesn't deal with most of that in some super serious way, property damage and civilian involvement for example isn't angsted over. So come on.
- Re: consent: most sane places take into account age differences, not just absolutes. So a 14 year old getting involved with another 14 yo isn't the same as with a 20yo. And rightly so, they're not the same. Also again, the whole "superhuman" and "fighting monsters" bits would probably result in different legal treatment too if it ever came up.
Plus, our protags here are villains (or at least on the villainous side, though villainy here has many sides). A certain amount of "evil" or at least amorality goes with the territory of course.
- Age and combat: if anything this should have been the bigger thing that'd come up around age. These are literally child soldiers. And child soldiers are still an unfortunate actual reality for much of the planet. First-world norms here are themselves pretty new, in the last few hundred years having kids along learning or attending combat units was still very much a thing. Magical girl series can raise (or not) this issue as something to expose 1st World audiences to in a more familiar context. Some actually consider the psychology, PTSD etc. Some just mostly blow it off, or else consider it primarily within the context of "this is why they don't do certain things an adult would", which matters from a story telling perspective since if you gave a random adult superhuman powers there is a good chance they'd use them quite differently.
Not every series has to do some huge deconstruction around all this. Get over yourselves.
edit: "ptsd" not "psd" duh