They all look like kids till you either see the village chief or Shinri takes his clothes off lol
Byeo-ru was also very sick as a child and still is as an adult even if he's a lil better than before. Being sickly or having a chronic illness that keeps you from being able to even walk can stunt your growth. He's also not even that old. If he was supposed to be in his 30s then I'd at least understand where you're coming from, even if I don't agree with how ya'll love to police art styles or character designs, but he isn't.
Either way, none of the characters outside the chief, who is clearly way up there in years, have any visible signs of how old they are. The art style is too simple and everyone looks the same age anyways. I think ya'll just love to complain about anything atp.
I would pay the author cash money for her to stop drawing the main guy looking 12 years old. Also they should get freaky with Shinri still in dragon form please please please please let's get that dragon DICK