He's an oni, adolescence would look different for them. So like they could only be in adolescence at 100 years old. Creatures that live longer have later adolescence. Like how a cat is able to get pregnant at 6 months but a human can vet pregnant around 8-12 years old.
Supernatural creatures aging at different rates is nothing unusual, the same can be seen in monster high. Draculaura is 1600 years old, but developmentally that's 16 for her. Frankie was 15 days old but developmentally that was 15 years old for her
You don’t even know if thats the case here considering this is Japanese i highly doubt it.
Wasted such a nice design just to make him a pedo.