It's easy to sit back and take pot shots after the fact, but for an adult to accurately write a child character is not as easy as it sounds.
Of course not. It's hardly impossible, though. And besides, that doesn't mean someone can't raise their hand and point out that they didn't go a good job in that case, though. That's the right of anyone who spent money on something and was legitimately dissatisfied with it.
And besides, this is a forum specifically for discussing fiction. What's the point of it if you can't offer opinion?
Or you could just, I don't know, let it go and enjoy the story for what it is instead of picking over every single little minor detail. It's fiction for crying out loud. You're able to suspend belief ...
That's just the thing: the number one job of a fiction writer is to enable that suspension of disbelief.
Now, while I'm on this tangent, allow me to retract one of my criticisms against Mr. Scanlon. I said he did a poor job of depicting Caliban as a death world, but at the end of the day the most defining aspect of that planet was its population of Chaos-warped beasts. Zahariel and Nemiel join up at around the half-way point of the Crusade to destroy these monsters, and it's fair to say that, by this point, the Order should have done a proper job of securing the areas around their headquarters. Ergo, it's quite plausible that he never really had to worry about describing the effect Caliban's eco-system would have had on people.
... that people migrated to this planet thousands of years ago, that the birth of what is essentially an extra-dimensional god from an alien species' indulgence in excesses is why this planet was isolated for thousands of years.
Because that's the setting, that's why. If the author starts off his novel with something like, "1985 was the hottest summer on record for New York City," you have no reason to dispute what the author is telling you. You're accepting certain premises with any sort of fiction. Suspension of disbelief is ruined when the author disregards the concepts and themes that drive the fiction.
In some cases, that can be something blatant. For instance, a lot of people dislike C.S. Goto's 40k fiction because it features Terminators who can do back-flips. This ruins suspension of disbelief for them, because Terminators have always been described as juggernauts who sacrifice speed and agility in favor of amazing protection. In this case, it's something more nuanced: nine years who don't act like nine year-olds.
Don't get me wrong, I do think that anyone is entitled to dispute Monty's opinion and make their case that the author does effectively describe them as appropriate nine year-olds given Caliban's setting. I don't think Monty or anyone else is out of line for bringing that up as an issue, though.
You don't seem to have any real problems accepting that The Lion managed to survive out in the Caliban forests as an infant.
Why should I? One of the central premises of this setting is that the Primarchs are superhuman beings with genius intellects and an innate understanding of how certain things work. They grow at an astonishing rate, and even as infants can be deadly.
Genetically engineered super-human or not, where did he find food, water, clothing, shelter?
"Normal" Space Marines can survive on normally inedible material, can go long stretches without food or hydration, and are able to easily survive through exposure and temperature extremes. Why wouldn't a Primarch?
How did he not get eaten by predators while he slept?
"Normal" Space Marines don't need to sleep for weeks at a time thanks to their Catalepsean Node. I doubt Primarchs are any weaker.
You don't have any problems accepting that The Emperor can basically psychically manipulate his own cellular structure to make himself effectively immortal and having lived for tens of thousands of years even by the time these stories are taking place. You're able to accept all kinds of other completely improbable things by today's standards, but you draw the line at kids sounding too adult? Really?!
Of course I don't have a problem with it. Virtually everything you mentioned above falls within things that are a given within 40k. If we used your mindset, then no fantasy or science fiction is plausible, since all of it fundamentally defies what we know to be real. It comes down to the parameters of the setting.
I have the obligation to accept certain themes and concepts of the fantasy I'm reading. For instance, Superman has amazing powers that are fueled by our sun. Achilles was made nearly invulnerable by being dipped in the River Styx. In this case, the 40k universe is a dystopia defined by god-like creatures that rule a parallel existence that is a psychic reflection of our own material universe; their chief opponent was a superhuman psyker who used advanced technology and (probably) sorcerous powers to create twenty other super-beings who were in turn used to create an army of super-soldiers. Again, if I refuse to accept these things, there's no point to any of this. Everything from 'The Iliad' to 'Star Wars' becomes a colossal waste of time.
For his part, the author has an obligation to stay operate within those defined parameters. Dismissing them (Superman is not affected by kryptonite; Achilles is stabbed in the gut) ruins suspension of disbelief.
In this case, Monty's complaint has to do with something that has to do with the normal human experience rather than a specific theme in 40k. He feels nine year-olds should behave like nine year-olds unless the author convincingly qualifies why they shouldn't. Is this a valid complaint? Why not? 'Star Wars' isn't a story just about the powers Jedi enjoy. The central conflict involved a generation story about ambition, fall, and redemption. If Luke Skywalker doesn't feel at all conflicted when he discovers that his father is the right hand man of a tyrant who is willing to kill billions to have his way, is a watcher not entitled to complain? Why is it different when characters don't potentially act either as kids that grew up within living memory of horrific conditions or even as normal children? And again, if you feel like those kids should be acting the way they do, that's fair - offer your counterpoints. I just don't think it's valid to tell someone to basically "shut up and color".
I mean, If a person can't expect his pleasure or disapproval over a piece of fiction on a forum designed for just that, then what's the point of said forum?