It depends on the novel. Most novels with teen characters (or slightly older) are YA. That's who tends to care about that age group, so it's easy to market. However, if an author (agent, publisher) feels the plot, writing, or issues would appeal to an older audience, there's no reason that the age of the characters has to define where they are placed. I think it might simply be a matter of where everyone involved feels the novel is best placed (or where they are comfortable in placing it).
That's just my guess, though.