I'm not going to vote this time, but I do want to put my two cents forward because I work with teens. Previously in the classroom, now in smaller music-lesson groups.
They are neither children, nor adults. They're adolescents, which, as any parent will tell you, is definitely not either category. (If it were, they'd be easier to deal with!)
Also, the variation in maturity levels between the ages, as well as between individual teens of the same age, is absolutely wild. For simplicity's sake, I'll generalise the age brackets, but I've taught 13-year-olds who still acted 11, and 13-year-olds who acted 20, in the same classroom. (It makes managing behaviour a blast.)
Their life experience will make an enormous difference in how maturely they behave as well. A teen who's helped raise 4 younger siblings while their parents worked late will be far ahead in maturity when compared with a teen whose mum has helicoptered around helping them with everything their whole life.
Also important - girls mature faster than boys in terms of their emotional control and self-regulation. And a non-neurotypical teen (ADHD, mild autism) will be a year or two behind their peers in emotional maturity, even if they're quite smart. (I was this!)
A young teen (13 - 15) is, on average, going to demonstrate more childish behaviours - though they're still distinct, behaviourally and psychologically, from children. They will struggle more with impulse control, care enormously about identity and friendship groups, and find separating reason from emotion quite difficult. They tend to have very black/white views of the world. They also tend to base their identity around things they like, and get very enthusiastic - and defensive - about those things.
A mid teen (16 - 17) is, on average, growing out of their childish behaviour. They're still quite black and white in their thinking about most subjects, but they're better able to handle nuance. They're also getting better at tolerating personal difference. Their ability to regulate their behaviour and feelings is improving. Generally, talking to them feels like talking to a young adult, albeit a very naive one with zero real-world life experience.
A late teen (17 - 19) is a young adult, though a childish and often still self-centred adult. They are mostly able to manage adult tasks and responsibilities, they're able to think in more nuanced ways, and they behave like adults. That said, most still lack life experience, so while they walk and talk like adults, they're going to be one who still has a lot to learn about the world, and they will probably still stumble a lot.
As for how I write them... it depends on a few factors.
- Age
- Personality
- Environment/upbringing
If I were writing a story about a 14-year-old in Australia, they'd be written more like a child than an adult. However, were I writing a story about a 17-year-old in, let's say Germany, they'd be written more like an adult than a child. The culture you grow up in, and particularly how much independence and responsibility you have growing up, has a big impact on how quickly you mature.
Finally, a change in environment is a massive boost for growth. A human being faced with adversity which they have to overcome themselves will either sink or swim, regardless of age. (Unless they're a very little child.) A teenage character faced with a 'save the world' scenario, who has the capacity to rise and meet that challenge, will mature through that far more quickly than they would otherwise, and will come out the other side far more 'adult' than child or teen. This is a big part of why teen characters in fantasy stories don't strike me as enormously unrealistic, because they will rise to the occasion. (Though personally, I think ages 16 - 19 are much more realistic for such stories than ages 13 - 15.) It's also why, real-world, travelling alone during a gap year or going away to college rather than living with parents is so important for cultivating maturity.
