@Varler The art style does make them look younger to an extent, but other details are correct for that age. The biggest one is Enji's breasts have budded (she's not flat), which happens at the start of puberty for human girls. She has a very typical body for a human girl that age.
@Chizan She's been put in the same grade as him, even though she's clearly smarter than many adults in some areas, (see chapter one, where she's reading books on advances physics) so she's clearly meant to be around the same age as him
developmentally and emotionally.. Whether she's
physically that age or not doesn't really matter for the story, at least not so far.