While looking at Does Not Like Men, what a person edited under the Kureha reference made me think about Mashiro in the beginning. In the first five or six pages on the entire series, you see him getting his period and I assume by him saying 'Impossible...' that it's his first one. I'd place his age around 16 or 17 and girls usually get their periods around 9-14. So, why is he just getting it then? And why the shock? I mean, if it was his first time I can comprehend some, but not to the point where it's unimaginable that he would ever get one since he was aware of the whole boy/girl thing.
When he first talks with the nurse and is told he must get the key to graduate, he says he is a freshman. I'd say this puts him around fourteen. Also, perhaps sie thought the male half would interfere with how the female half worked.
There's not a lot of point in wondering about it, since the nature of Mashiro's affliction is totally made up and anything he was told about his condition would be equally fictional, if the nature of the world even provided that moment in his memories to draw on.
Mashiro could be very much like Akito from Fruits Basket She was born a woman and raised as a man or s/he could just be a very confused flat chested girl... Has anyone besides me thought on that?
Echoing one of the below tropers, this is technicaly a fake world and Mashiro didn't actualy have any real life before that moment. According to her nurse, "you didn't need anything else."
Apparently the average age of first menstruation is about 11-13, and it is definitely not unheard of for it to occur much later. Saying "impossible" doesn't really make sense since she/he must have been aware of her/his own biology (although as others have pointed out, since it's a fake world, he/she may not have questioned or realised if nobody ever told him/her about that), but I would say the feeling was more like "no way" or "this can't be happening" - a sense of denial.
I should think that having been without one for at least two years past the average, it's a reasonable guess that Mashiro concluded that s/he wouldn't ever have one.
For that matter HOW the hell did s/he get diagnosed in the first place? I mean at birth only the gender of the lower half would have been obvious, so why the hell wasn't Mashiro raised as a girl? Do the Japanese karyotype children at birth or something?
Well, considering that it's revealed that everyone's living in a fake world anyway, this troper just assumed that whoever it was that "birthed" Mashiro brushed it off...then again, this is a Mind Screw series.