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.
Another option is Wattpad. There are many Naruto and Nezuko fanfictions on there as well. Some of the stories on Wattpad are more focused on the emotional and relationship aspects between the two characters, while others might be more action - oriented. You can explore different genres and writing styles on this platform.
First, decide which universe you want Nezuko to cross over into. Then, think about how her character traits and powers will interact with the new world. For example, if it's a crossover with the Fairy Tail guild, consider how her demon nature will be perceived by the mages there.