It think it was either Steam or Patreon (or both) that would have banned the game if Chiaki wasn't and couldn't pass as an adult. Which is why she is not only older, but dressed differently.
Viewing post in 0.5.9, New Chiaki, Steam, some good news comments
i do believe it was stated in game that she was in elementary school. So yeah, she was underaged; making me think we weren't going to have a sex scene with her. But this being a porn game, it would make some people uncomfortable whether anything happens or not.
Sucks for people who're into that though. No kink shameing