One popular 'zenitsu x nezuko fanfic' is 'The Unlikely Bond'. It beautifully portrays the relationship between Zenitsu and Nezuko. The author does a great job of showing Zenitsu's protective nature towards Nezuko and how Nezuko's presence calms Zenitsu in his often-fearful state.
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.
There is a 'zenitsu and nezuko fanfic' titled 'Under the Cherry Blossoms'. In this story, the setting is under beautiful cherry blossom trees, and it's a really romantic take on their relationship. Zenitsu is shown to be very shy yet determined to be close to Nezuko. Another one is 'Nezuko's Soft Spot for Zenitsu'. It shows how Nezuko, despite being a demon, has a special place in her heart for the sometimes - comical Zenitsu. These fanfics are great reads for fans of the two characters.
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.