I’ve noticed that main characters, even in adult fiction, tend to be young. Even when they are not young, they are often prone to teen/ young adult traits that foster conflict. Traits like impulsiveness, lack of censorship in dialogue, rebellion, and identity crises. All of these are universal traits that people experience to greater or lesser degrees throughout their lives but are distilled – exaggerated – in youth. This distillation, above all, is why I think books tend to use young people as protagonists. But these traits are useful and realistic in characters of any age, so take a look even if you write older characters.
Teens and young adults are impulsive. They are more likely than their older counterparts (generally) to act before they consider the consequences. Many parents lament over the trouble their children (and not-quite-children) land themselves in because they didn’t stop to think. But for the purposes of a book or poem, this is a positive trait.
This ties into impulsiveness. Young people (especially younger
people) speak without censoring what they say. They say what is on
their minds and they speak without regard for how their words will
make other people feel. This is a valuable quality for generating
conflict in dialogue.
Many teenagers and young adults question social conventions, take up
causes, and find themselves feeling like outsiders. All of these set
up conflict because they are not living in accordance with
pre-established rules and expectations. At some point they will have
to stand up for their beliefs. Conflict!
Many protagonists are in the process of learning who they are. This
makes for juicy conflict with loved ones, society, and himself. Even
the past and future are in conflict, with the protagonist’s present
caught in the middle.