Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
After the initial shock and sadness, Twilight often becomes determined. She decides that she will not let the betrayal break her. In a story where she is abandoned in a dangerous realm, she uses her magic and wits to find her way back and also to expose those who betrayed her, showing that she is stronger than they thought.
A simple Nezuko and Zenitsu fanfiction could be that they go on a journey together. Along the way, they encounter other demon slayers who are initially skeptical of Nezuko. But Zenitsu stands up for her, explaining that she's different. This shows the depth of their friendship and how Zenitsu has come to trust Nezuko completely.