haha, I could see ryo saying "its not what it seems" but the picture is actually in reference to when ryo comes back to visit her (Ryos promise, which has never happend since there are no other shenmue games) thus I would assume she would finally be old enough at that point
I do think she's 15 though, I suppose only if you got the birthday scene.
But anyhow, I got the idea from the good bye scene where fangmei saids the following
"good luck... please take care... go to bed early...and..........I love you. Please you dont have to say anything at all, but someday come back here, ill be waiting, I believe in you (ryo: I promise I will)