Never liked this concept. It's far too generalized and ends up confusing people.
Let me break this down, even if you understand this already:
What we think of as the author's "voice" is their style of writing. The vocab they know and would use often, they way they word what they say, the way they order it, the way they use it for a singular purpose, how it looks in the big picture, ect. Pretty much, your personal communication skills as a storyteller and writer.
Then there's the narrator's voice. And yes, it's a voice. The writer inputs a style onto the page, and the reader outputs that style as voice--because they hear it in their head. Now, I want to make clear that I don't mean the first person narrator. If you have narration, you have a narrator. First person is when the narrator references themselves in the story, third person (and second, sometimes) is when the narrator references other people. Most of the time, the third/second person narrator is a character--like the first person narrator might be--but they can also take on certain attributes of the POV characters they are covering.
Now, I'll take this into YA:
The majority of the books in YA are written in first person for a number of reasons, and I'll agree that the emphasis on the YA narrator's voice to be key. YA the primary genre that I know of that emphases one POV style over another (though, of course, third person isn't uncommon, nor ridiculed, just not as popular). With third person, IMHO you'd have to focus on the character narrator's specific voice or expand on the POV character's style of voice within the narration.
But that's the thing: The narrator's voice, the character narrator's voice, the POV character's voice. There is no "YA voice" that people should write, but your character's personal voice. How would that character, narrator, or POV would say it? Does it sound like a young adult would be speaking it? And not some 42 year old woman sipping her coffee and putting in her 1500 words for that day?
And that... yeah, defining the "young adult's voice" can be hard without falling into cliches and overused tropes. Their characteristics--which would define them as young adults--should be represented in both their actions and reactions in the story; and commentary and diction within the narrative. Those, in whatever which way, should give evidence that the narrator/character is a young adult who's in adolescence--and that, is up to the style you focus on and how the reader interpenetrates the voice.
