
is no one else bothered by the fact that hes literally mentally 10 years old despite having an adult body

more like emotionally & socially stunted due to his isolation for years but he's still mentally mature with how he discussed his reason for saving Gitae and all. I've seen HS students like Jihun & most of the time, they just need to be exposed to people and socialize so they can catch up with their peers

He is not "mentally" 10. It's more like he is socially and emotionally underdeveloped. Not everyone develops all of their capacities at the same pace as their physical age. For example, a Sociopath does not develop empathy, therefore his emotional capacities are underdeveloped. A kid partially or completely cut off from society won't develop their social capacities as efficiently as others. The problem with romantic relationships between people with different capacities resides when the capacities at play may put the affected party in vulnerability and danger of being exposed to something their development is not ready for, and therefore being abused, specially when the areas are sexual, physical or cognitive capacities. A few examples, sexual underdevelopment may come from being sexually abused at a short age, and the person may stop the development due to trauma, physical can be caused by a specific syndrome or set of conditions, as well as cognitive underdevelopment. I personally had a unique set of circumstances which made me socially underdeveloped and sexually and emotionally overdeveloped for my physical age, at the end we are all unique individuals and our characteristics may vary from person to person. The goal when forming a couple is not to choose a person whose central capacities are not astronomically different from yours, since it may vary the power balance inside the relationship and increase the probability of abuse to surge.
i feel like i find it hard to like this story because there is no established world building, the story only takes place within this castle and nowhere else and the plot is also very out of context? its just so hard to relate to any of them