In the "Demon Slayer: Kimetsu no Yaiba" series, the exact age of Nezuko Kamado, the sister of the protagonist Tanjiro Kamado, is not explicitly stated. However, at the beginning of the series, Nezuko is depicted as a young girl who is a few years younger than Tanjiro. Tanjiro is portrayed as a teenager, approximately 15 years old at the start of the story. Therefore, Nezuko is estimated to be around 12 or 13 years old in the early parts of the series. It's worth noting that the passage of time and character ages may vary as the story progresses.