Sorry, I don't have the specific name of the author of the Ennead comic. Maybe you can look for it on popular comic forums or websites dedicated to comics.
The best place to look for the ennead manhwa raws is on official manga sites or authorized distributors. However, if those don't have it, you could explore some lesser-known manga aggregator sites, but always be cautious about the legality and potential risks.
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.