Rina is of age infact she tells her age sometime in the beginning before the handjob however the game doesn't always like to show it when you go to her profile it's a small big or glitch but it should fix itself after awhile if not hopefully you made a save before then all you have to do is reload that save but if your too far in to want to go back just keep playing I think she mentions her age again or maybe not I don't remember but she is 20 years old