An adult and a girl are two different things. But, I agree. Too bad she's not married though. That would be hot.
Actually, "girl" is a diverse term. From what I know, to specify, you can say 'younger girl' to refer to girls who have just hit their puberty and 'older girl' to the ones who are in their adolescence years (20-40) However, it's a different case with males. Normally males who are above 20 are referred to as 'young man' or 'bachelors' rather than "older boy". And if she were married, the game would have been about NTR, which some people might not have liked. Either way, in my case, if the adult game has female protagonist and I can corrupt her however I wish, then that's a green light for me