In materials collection Zoisite is 16 years old . Why Kunzite is 25 . In /japan you're not an adult until your 20 . So by in there standards he is child.
Well the phrase in question was used for children and pets, and teenagers aren't particularly thought of as "children" in the same cutesy sense. At least that's my understanding.
I think something that a lot of people haven't considered is what the Materials Collection is supposed to pertain to, exactly. Many consider its information to refer to the continuity of the manga, yet it contains points and notions that were only ever true for the anime (i.e. - how Shingo feels about Mika, who wasn't ever in the manga). Likewise, it also features characters who were only in the SuperS movie, a film that, while not neatly connected to the continuity of the anime, bears stronger ties to the anime than it does to the manga.
Thus, with the book's contents feeling more like a blend of both the manga and anime, I feel that it might not apply solely to either one of them, but that its information instead served as a background source that BOTH the manga and anime might have picked selective bits of info from, but with neither the manga nor the anime completely adhering to everything the book contains. As such, the book itself may be viewed as its own unique continuity that is neither solely the manga nor solely the anime, but instead a "master continuity" containing elements of both that each one took from the info published in the book.
Therefore, whatever the book says about Kunzite's relationship with Zoisite may not necessarily apply to either the manga or the anime.
I didn't know the collection mentioned Mika! It seems like Naoko had an awful lot of ideas she didn't get to explore, it's kind of sad.