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.
"Criminal Law of the People's Republic of China" article 20 provides for the situation of justifiable self-defense. According to this article, in order to prevent the country, the public interest, the personal, property and other rights of the person or others from being violated, an act taken to stop the illegal violation and causing damage to the person who violated the law is considered justifiable defense and shall not be subject to criminal responsibility. This means that under legal circumstances, individuals can take necessary actions to protect their own or the rights and interests of others without incurring criminal responsibility. However, if self-defense clearly exceeded the necessary limits and caused significant damage, criminal responsibility may be incurred. In addition, defensive actions taken in the context of violent crimes that seriously endanger personal safety, even if they cause casualties to the wrongdoer, are not considered excessive self-defense and are not subject to criminal responsibility.
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.