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.
Some of the clones might feel guilty, thinking that it was their fault she got injured. They would do everything in their power to make her comfortable while she recovers. They would offer her food, water, and anything else she might need. They would also be very protective, forming a sort of perimeter around the area where she is being treated.
A simple Nezuko and Zenitsu fanfiction could be that they go on a journey together. Along the way, they encounter other demon slayers who are initially skeptical of Nezuko. But Zenitsu stands up for her, explaining that she's different. This shows the depth of their friendship and how Zenitsu has come to trust Nezuko completely.