Writing sexual fanfic, especially when it involves non - consensual or inappropriate use of characters, is not appropriate. It goes against ethical and respectful creation of fan - made content.
I don't know exactly. It could depend on the specific manga universe or storyline. Maybe you can search through popular manga forums or fan communities for the answer.
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.