Tanjiro age at the start of the series: 13 years old
Tanjiro Kamado, the protagonist is 13 years old at the start of the story when his family is attacked by Muzan. After the 2-year time skip, he ages to 15 and becomes a Demon Slayer. He remains at this age for most of the story. You'll finally bump into the 16- to 17-year old hero by chapter 204.