Yes, there are some. You can check fanfiction websites like Archive of Our Own. Just search for 'naruto x nezuko' in their search bar, and you'll likely find a variety of stories written by different authors with different takes on this crossover.
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.