They do act their age, which is about 10. They didn't transmigrate, they were reincarnated i.e. they are new people with the memories of their old lives. Which explains why they're oddly mature but at the same time they are still just children. People aren't flesh robots, the hormonal balance alone would throw you off never mind trying to reconcile what is basically a long dream with your current new existence. Every one of these stories always has plenty of "Oh my, why aren't they acting like the cold rational creature of logic that I am? Lowly human failings disgust me!" in the comments.
That said the most recent chapters do nothing to ameliorate my earlier criticisms about how disappointing the twins emotional about face is (and what garbage everyone in the story is)