When characters are very different than the people in your own day-to-day life, finding the right words to put in their mouths can seem like a mystery. Building a voice—which is what you’re doing when you write dialogue—is an aspect of characterization. In order to write with authenticity, you need to know the characters well.
Start by getting an idea of how this age group sounds in real life before you begin inventing. Immerse yourself in the voices that you want to create. You might find public places where young people gather and keep your ears open. Watch relevant documentaries. Or turn to your own family. Do you have relatives this age? If not, ask around. Perhaps someone you know could put you in touch with teenagers you can interview. Or volunteer at a junior high or high school.
As you look for sources, keep in mind that background, location, and circumstance will make a difference in dialogue. If you’re writing about a family firmly situated in the affluent suburbs of Chicago, listening in on a group of students in the Bronx won’t do the trick. The language will be different, as will the colloquialisms, and frames of reference.
As you listen, pay attention to popular phrases, syntax, and diction. Also, mine the conversations you hear for what you can learn about interactions at this age. What goes unsaid? How does this age group respond to authority, conflict, or praise? Of course, everyone is different, but these observations can help you identify patterns that may authenticate your characters and their dialogue.
After you’ve spent some time listening to and thinking about these youthful voices, you’re bound to find that your particular characters’ voices begin to emerge. The more you practice, the better. Consider writing passages in first person from each of the characters’ perspectives. These won’t end up in the novel, but they will give you an opportunity to work out the kinks and truly hear—and master—their voices.
Authors have written entire novels in youthful voices. J.D. Salinger’s Catcher in the Rye and Russell Banks’ Rule of the Bone are just two examples. Don’t get discouraged. This can be done.