Debatable, haven't seen anything in game addressing the age, so no real way to tell (granted I'm using machine translation, so somewhat hard to tell). Also keep in mind that just because she doesn't have F-cups doesn't means she's underage. Up to the OP/mods what to do.
And the mods generally er on the side of "The supporting character is completely flat, therefore loli". It doesn't matter what the actual age is, for the most part, it's the appearance of the character.
The character that you think is loli has the exact same face and height as the third character, except (much) smaller boobs. So either both of them is loli, or neither of them is. You can't have one loli and one non loli just because the other one has bigger boobs.
We don't usually see games where the two character look almost identical. This artist just suck at drawing.
Looks like she'd be an A-cup to me (well she would be if she wore a bra). I do agree with the other guy who pointed out that she has similar facial structure and height as the other character with the G-cups (or are they H's?). Pretty sure we're getting derailed on this though, so might be best to just drop it and let the mods decide.
