I'm reminded of some comment Stephen King wrote about why he put off writing the fourth book in his Dark Tower series so long, and it basically had to do with the story involving the main character's first love as a young teenager. King wrote that he put it off because while he could certainly talk about adult love, he wasn't sure he really remembered what it was like to have the raw intensity of feelings you might have as a teenager experiencing the emotion of love for the first time.
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. It's like asking a 6 year old to write a story about someone who's reminiscing on their death bed after having lived a full life. There is no frame of reference. Odds are if the author had even tried, it would have come off seeming very hollow or contrived, not authentic in the least. Then people would be pissing and moaning about that instead of how the characters seemed too adult.
If it helps, consider that we're talking about people who have been living on another planet, completely isolated from all other contact with anyone else, for thousands of years. If you just look at fairly recent history on this planet, the diverse cultures that developed when they were isolated from one another by geography. That was generally only over the span of a few hundred years.
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. I feel sorry for people who can't just watch a movie or read a book without constantly pointing out how this or that detail is wrong or some action sequence is completely impossible. It's fiction for crying out loud. You're able to suspend belief 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. You don't seem to have any real problems accepting that The Lion managed to survive out in the Caliban forests as an infant. Genetically engineered super-human or not, where did he find food, water, clothing, shelter? How did he not get eaten by predators while he slept? 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?!