Youthful main characters aren't necessarily a drawback. ZZ Packer's short story “Brownies" is about Laurel, a girl in a Brownie troop that's preparing to beat up another troop at Camp Crescendo over a rumored racial slur. The main character of Frank O'Connor's short story “First Confession" is Jack, a peppery boy afraid to give his first confession. Both these characters are young—close to the age you're concerned about—but the stories don't suffer for it. The characters are deeply drawn, the conflicts are strong, and the stories explore engaging aspects of human nature.
How you handle the narration will make the difference between a riveted reader and a discouraged one. An engaging voice, for example, can keep a reader rapt. But if you're worried your character's youthful voice might lose its charm over the long haul, you can use third person to keep the perspective of the young character and, at the same time, have access to a larger vocabulary. You can do this in first person, too, as ZZ Packer does in “Brownies." Laurel's troop—girls from the south suburbs of Atlanta where white people were like “baby pigeons: real and existing, but rarely seen or thought about"—is on their way to the bathroom, where they plan to confront the white troop that is rumored to have slung the racial slur. Laurel is apprehensive:
No one talked about fighting. Everyone was afraid enough just walking through the infinite deep woods. Even though I didn't fight to fight, was afraid of fighting, I felt I was part of the rest of the troop, like I was defending something. We trudged against the slight incline of the path, Arnetta leading the way.Packer uses past tense and lets an older Laurel narrate in more sophisticated language (“infinite deep woods"), but keeps the reader focused on youthful Laurel's immediate experience.
Whether you dramatize moments from the character's younger years depends upon their relevance to the unfolding story. Some conflicts span a few hours, while others take several months or years. If you were telling the story of ten-year-old Debbie's summer at camp dealing with a malicious counselor, the five-year-old material would be background. Still, that doesn't mean flashbacks are out. For each memory you consider including, ask yourself: does the reader simply need to know that it happened, or is it important the reader see how it happened? For the former, you can use summary or suggestion and stay anchored in the unfolding action. For the latter, a scene would be appropriate. On the other hand, if the story is about Debbie growing up while weathering the disintegration of her family because of her parents' abusive relationship, the younger material may be part of the conflict, which means it's fair game for dramatization.
Determining the scope of the conflict can be tricky. Keep in mind that conflicts in fiction—and in life—are often slices of larger conflicts. ZZ Packer's “Brownies" is about racism and meanness, and one could draw from a character's lifetime to illustrate that. But Packer chooses to focus on the specific conflict of the two Brownie troops at Camp Crescendo and the rumor of the racial comment that sets it off. Novels often have a broader scope, but that doesn't mean the conflict loses definition. Jeffrey Eugenides' Middlesex spans generations to tell the story of gender confusion and the gene that caused it for the main character Cal, a hermaphrodite born and raised as Calliope. Pinpointing the focused conflict will help you make decisions about what to include and how to present it.