You were 12 before being turned into the first Demon King. Along with your younger brother, Muzan, who was 13; He’s also a demon. Soon after you left Japan and left your brother some of your blood, making him the Demon King after meeting Yoriichi. You can withstand the sun, you’re able to eat human food, and you don’t go berserk. 1014 years later, you went back to japan. It’s currently the taiso era. You are 10,000+ years old, but you are 21 years old in human years.