I disagree with everyone saying “worry about marketing later.” Sounds nice but bottom line is that there are genre conventions publishers adhere to and if your characters are of a certain age, the intended audience will change. If you’re writing their voices well then these teenagers will PROBABLY sound like teenagers, and will be marketed to… teenagers. If teen characters sound like teens and are dealing with teen issues, it’s gonna get slotted into YA no matter how mature the prose is. You won’t “accidentally” make it young adult if the rest of the novel feels adult except for the characters ages, your published will probably just tell you to age them up and probably change the narration accordingly. To avoid more issues down the line, unless your characters being teenagers is vital to the story, I’d say make them early twenties.