Perceived age of characters is a tricky thing. Since I'm currently 41, it's been a long time since I was a teenager. My teenage years aren't exactly memories that I cherish, either. My main character Kate starts Disciple at sixteen years old. The story will finish before she turns eighteen -- or not long after, at worst. I do not consider Disciple a YA story, though I don't doubt that teenagers might find it interesting.
Kate has been confronting me with questions like what is "maturity" and how is maturity demonstrated and what is "appropriate" for a given age and level of experience -- partly because she starts out "more mature" due to the cultural expectations put on her, and partly because she goes through so much in the course of the story.
In the course of preparing Part I to publish and revising Part II, I got some whiplash from meeting the (slightly) younger, (much) less experienced version of Kate.
It's possible some of them were sober, hard-working, kids who never questioned that this was what they wanted. Maybe they welcomed the responsibility of a job and felt it was a validation of their worth.
People come in all flavors, after all, at every age.
Getting back to Disciple, I'm more worried about Kate sounding like a 40-year-old than about her sounding like a 16-year-old. It's my opinion that the maturity level we perceive as "appropriate" for a given age is highly dependent on culture and affluence. But, as has been reported in a variety of (fascinating) places, the teenaged brain is noticeably different from the adult brain, and that needs to be taken into account.
How those difference manifest is heavily shaped by culture and affluence -- which is why I brought up the child labor photos. Compare those kids' lives to a well-off high school grad who's working a part-time summer job because s/he wants to, not needs to. People talk about how kids are in such a hurry to grow up (usually with a shake of their heads). I doubt anyone said that of the child laborers.
This post is already turning into a ramble. In short, I feel comfortable with Kate "sounding" older than 16 because in her culture, she's expected to be married, a mother, and running a household at that age. Due to the story, she's actually responsible for much more than that. And yet, I can't have her acting with all the mature perspective (lol) of a 40-year-old.
What does your character's culture expect of children and teenagers?
See also: Demonstrating maturity