"Mature" is a relative term.
40K Literature has a broad depth to it. From over the top combat, black comedy with the Orks, to complex character sci-fi with the Eisenhorn series.
What makes the marketed for children aspect so bizarre is that the 40K Universe is a Heavy Metal album cover given life.
It is a horror universe. The anti-Star Trek. 40k is where humanity is in control of a vast interstellar Empire which persists on xenophobia, fear, hatred and ignorance on a massive scale. However it is so entrenched and and its enemies so much more horrific in comparison that any attempt to reform it may doom all mankind and turn the galaxy into a literal hell.
You have weapons and demonic powers which bring body horror to new limits. Necron flayer weapons which House Bolton people on a grand scale. Tyranid devourer fire creatures that burrow into the flesh, eating their way through the victim's nervous system towards the brain. The agony caused by this process is enough to drive the victim insane. Not to mention chaos sorcery, the nightmare of Nurgle's Rot. Soldiers trapped as screaming faces on the armor of The Dark Eldar who have perfected torture to such a degree so they can drink their victims souls at the height of their torment.
The mainstream 40K universe is not a place where these kids would survive long. It's one where their antics would be declared as heresy, then they would be burned at the stake and the peasantry would applaud the foaming mouth preacher condemning them for his diligence and fervour. Assuming their naivety didn't get them killed first.
It has spawned some hilarious memes.
If the Warhammer Adventures books are their own separate universe it's fine. 40k Is generally separated into its three different aspects of hobby, tabletop and lore. Games Workshop stores are usually filled with teenage boys but the private gaming clubs outside of those I've been a member of were generally 20-50 year olds.
The big concern is that GW's trying to kidify the main universe when 40k's over the top and Grim Dark approach is what make it fun, unique and interesting. It's already had to suffer Matt Ward's terrible fluff. The nonsense with how the Primaris marines were improved over the Emperor's own and how they would even get along with all the established first foundings various quirks and dark secrets.
Thread ends here. Cut along dotted line.
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