To begin with, I want to abandon the term 'juvenile' as a genre title. Firstly, it carries too many negative connotations. Secondly, it abandons the existing genres of "children's literature" and "young adult fiction". I understand the idea of categorizing works of art is itself contentious. In a series on modernizing genre, I discussed this conflict extensively. To quickly summarize, I believe current videogame genres are largely useless. Genre itself is still useful however. Genres offer exploratory maps that foster cycles of interpretation, design, and analysis. This creates a valuable conversation across time between consumers and creators. With the use of mechanically descriptive sub-genres, videogames should embrace thematic genre classifications.
That being said, categorizing a 'young adult' game is particularly challenging. As David points out, books are similarly difficult to encapsulate with easily understood monikers. The Harry Potter books, for example, are more intellectually rigorous than many airport-available mystery novels. Likewise, while The Cement Garden features young protagonists, it is most certainly not a children's Novel. David states:
"So, while I can come up with ways to tell that a books isn’t juvenile literature (because of the style of language, because of sex, because of certain other topics), I found it surprisingly difficult to come up with a positive and non-banal description of what it means for a book to be juvenile literature. And that carries over to video games as well."
Videogames are burdened with another barrier to simple classification - gameplay. A children's book may be judged poorly if it includes excessively esoteric vocabulary or inappropriately adult themes. A narrative driven game built for children can also be criticized for including unnecessarily difficult or complex gameplay. This is not to say exactly how difficult a game should be, but these concerns should be taken into account while judging young adult and children's games.
There is no easy way to know what makes an adolescent videogame - which is part of the fun. Genres are not well defined outlines. A genre is a commonly recognized suite of thematic and structural decisions shaped by the creator's intentions. A skilled auteur may toy with conventions, including and abandoning them as she sees fit, responding to and building upon a genre, while commenting on society and culture along the way. As such, for some analysis, it may be useful to divorce game mechanics from narrative themes.
The mechanics of a children's game should be appropriate for its target audience. A younger audience may not have the educational or gaming experience to easily interpret and implement complex mechanics. Similar to literary reading levels, this does not preclude adult enjoyment. A game may have many mechanical layers, satisfying a young and old audience at the same time. Alternatively, simply mechanics can be implemented in robust ways by an older gaming audience, giving them complexity without confusing the younger players.
Similar to literature, young adult and children's games should share common thematic elements. We should see literary conventions repeat themselves within their videogame genre counterparts. As David points out, many kids books include young protagonists, often coming of age or having an adventure free of familial relationships. Again, this need not be the case. These are merely several, albeit very common, elements from a suite of options. Mrs. Frisby and the Rats of NIMH is a notable literary exception from the young protagonist trope. It is a children's novel with an adult protagonist and themes of parenting. It does, however, use many genre conventions to its benefit. Children's games can achieve this same diversity while safely building upon an existing body of work.
The thematic elements of one genre can absolutely bleed into another. I have discussed the 'lack of family' trope in videogames once before. Similarly, the 'coming of age' tale shares storytelling characteristics with the common game structures that gradually empower players. So, while I agree with David that Bioshock has similar genre characteristics, its adult themes and mechanics firmly separate it from the young adult genre.
Zelda, on the other hand, is certainly a young adult game. I'm in agreement with David when he says "they're about boys growing up (literally, in the case of Ocarina), forced to be men a little earlier than they’d like to, but rising to the occasion, finding out who they really are, finding unexpected depths inside themselves." Zelda informs other young adult games, and can be analyzed alongside other young adult novels like A Wizard of Earthsea and Sabriel.
Accordingly, we can analyze and critique games with their genre in mind. Lucidity, for example, is thematically a perfect children's game. It's mechanics, however, are far too punishing for success amongst the appropriate demographic. Final Fantasy VIII is an excellent young adult story with a great deal to say about family, friends, school, and growing up. Like J.M. Barrie's novelization of Peter Pan however, some of the more adult themes may be inappropriate for younger audiences.
Genre boundaries are porous, videogame genres even more so. The relative age of the medium is such that no broad repertoire of masterful children's and young adult works exist. While there are plenty of childish games out there, few are considered classics. Children's games as they exist now, including MySims Agents, are largely discredited. Children's literature has overcome some of these prejudices and the videogame medium must do the same. Game's like Little King's Story, seek to break the mold, entertain multiple age groups, and contribute to a larger cultural conversation about growing up and the lessons that come with age. With genre in mind, games and games criticism only improve.