The gradual development of a teen character who gains power and influence is important, because many teens feel they have no power over their lives. Vicariously they want to read about archetypal teens who develop self-determination and independence, and maybe get some analogies to their own development. Some of the strongest characters showing progressive development were Luke Skywalker in the earliest STAR WARS trilogy, and now Harry Potter, and these just HAPPENED to be franchises with a runaway success among teens. I wonder why?
It's the same with fantasy role-playing games; teens love to gain "experience points" or increases in stats, or have a character develop political stature in the fictional role-playing environment. Why? Because they may not experience such a clear-cut increase in power and influence in their real lives.