I'm not going to speculate on authorial intent because there's more than one author and I can't read one person's mind, let alone many. However, the way I see it is maturity isn't a linear, one variable thing, where you are more or less mature based on age. Iruru is more mature in some ways, and less mature in other ways; she has spent less time on Earth than Kanna, but is overall older. She is obviously more sexually mature than Kanna (being obviously interested in a boy, whereas Kanna doesn't seem very interested in romance yet), but has pretty poor emotional intelligence compared to her. Also, just because she had a fight with Kanna doesn't mean she's like, on the same level as her--if you have a little sister, it's perfectly reasonable to have a fight with her.