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.
A common theme is the power of their combined strength. Tanjiro has his own set of skills and Mitsuri has hers. Fanfics often explore how they use their combined powers to defeat powerful demons. Additionally, the theme of hope is often present. Their relationship gives them hope in the dark world of demon slaying, and they inspire others around them with their positive attitude and connection.
One great 'tanjiro x mitsuri fanfic' is 'A Bond Beyond the Demon Slayer Corps'. It really delves into their relationship, showing how their different personalities complement each other. Tanjiro's kind - hearted nature and Mitsuri's energetic and affectionate side create a really interesting dynamic in the story.
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.