Seems like a lot of people commented without actually reading the post...
Anywho. If you want to sell your game on Steam, then you'll have to comply with their rules. If Valve says, "No characters under 18, regardless of what they do in the game," then you'll have to age-up Chiaki in order to sell on their platform.
She doesn't have to be lewdable just because she's 18, but... People will want to be able to.
BTW, the adult Chiaki concept looks cute as hell.