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.
Once upon a time, in an old, creaky house, there were strange noises at night. Every midnight, a scratching sound would come from the attic. One night, a young girl decided to investigate. As she climbed the rickety stairs, the noises grew louder. When she reached the attic, a cold wind blew and she saw a shadowy figure. She screamed and ran out of the house, never to return again.