Is this really any different from lots of other media though?
I think the difference is in the tone that's being used. Shonen Manga tends to focus on pre-adolescent or adolescent kids because, well, it's the intended market. The problem I have with Shonen, though, is that most works in that genre stick pre-teens and teens in situations they wouldn't be ready or able to deal with in any shape or form.
My grandmother talks a big game about how she was actively working by the time she turned sixteen. My father started at fifteen and actually lied to get hired earlier than legally permitted. Nowadays, some kids don't enter the job circuit until very late in their teens, sometimes early in their twenties. That goes without considering the remaining effort to get into an actual *career*, at which point you can stretch that out to the early-to-mid thirties.
So when I fire up Resident Evil the first in name and see that Rebecca Chambers is something around sixteen or seventeen and is ALREADY operating in the Resident Evil-verse's most common Spec Ops-alike team, I kinda cringe. I know it's a Japanese thing, guys like their girls young and nubile and all - but it really breaks credibility. At sixteen, I would've started whining like a five year-old if you'd asked me to start planning for my retirement or to pay taxes!
I got sidetracked, here. So, I have issues with the Fantasy Employability of teens and preteens as typified by Shonen Manga. I think it could also be considered a leftover of old playtime routines. Children like to pretend they're adults, and kids like reading or watching stories where they still are young and hip and cool, but somehow have the weight of the world on their shoulders and Super Important Serious Jobs.
Like, um, piloting giant mechs or slaying sexless humanoid cretures that stand thirty feet tall. Yep, that's absolutely serious. Keep in mind, I've watched Anime as much as anyone else. I can pocket that peeve of mine and just enjoy a series for its own merit, but I'll confess I couldn't do that with Attack on Titan. That's another story.
As for Ender - I feel the aim here is to show that Humanity is so desperate they have to keep pushing the drafting age backwards. This doesn't really make sense to me. I mean, they must have veterans in that universe, right? Why not pull their hypothetical expy of the Red Baron out of retirement? Why not bait all the recently retired officers with killer bonuses? They could provide assistance to the families that had to sacrifice their seniors to push the Formix back, too.
But nope, child soldiers are a go. Increased neuroplasticity and faster reaction times being the cited reasons behind that; which, to me, is total bullshit. Not every senior has a ticking time bomb that explodes and triggers Alzheimer's; I've seen eightysomethings that had better reaction times and actual reflexes than kids half my age!