I think that he doesn’t quite act like an adult. I just read him as a precocious kid. It would be cool to see some isekai where the characters don’t act like adults, but process things like children who just have a lot of extra info.
Also, adults make bad decisions too. Every bad or weird decision made in isekai like these doesn’t only happen because the main character is “acting like a child.” It’s usually because the plot and characterization are weak.