This paper reports that a girl in Myanmar spontaneously changed from female to male. Is that even possible?
Spontaneous sex change does not occur in humans.
External genitalia are often ambiguous, the degree depending mainly on the amount of testosterone produced by the testicular tissue between 9 and 13 weeks of gestation. However, there are multiple classifications of hermaphroditism including simultaneous and sequential. Simultaneous hermaphroditism is when an adult organism has both male and female sexual organs at the same time. Sequential hermaphroditism, as described in the Taipei Times article, is when the individual is born one sex and changes sex at some point in their life. Protogynous hermaphrodites (as opposed to protandrous hermaphrodites) refer to organisms that are born female and at some point in their lifespan change sex to male. As the animal ages, based on internal or external triggers, it shifts sex to become a male animal. It is generally triggered based on mating opportunities related to reproductive success. It is observed in fish, almost exclusively.
Sequential hermaphroditism, especially of the kind described in this article, cannot occur in humans because the genitalia are formed in utero between 9 to 13 weeks gestation. The kind of sequential hermaphroditism described in the article has never been observed in humans. This condition can be found among a few species of vertebrates, mostly fish; and some species of crustaceans, including barnacles and shrimps.
Wartickler's answer sums up why a sudden over night sex change can't happen. I want to address a possible explanation to the event.
The only evince we have that the change has happened is that Than Sein's (the person in question) birth certificate states he was born a female. This can be easily attributed to him being an intersex1 person, as the article implies:
People privately concede Than Sein is a hermaphrodite. Several medical experts have examined him, and he awaits test results from the central women's hospital.
In some cases intersex people have ambiguous genitalia, even as adults, so a "mix up" on the birth certificate of an intersex person, especially one written in Myanmar 21 years ago, is very plausible. The same happened for example with Sir Ewan Forbes, who was registered at birth as a female, but later formally re-registered his birth as male and married.
1: I realize that the issue of how to call people with sexual anatomy that doesn’t seem to fit the typical definitions of female or male, is still going on, to the best of my knowledge "intersex" is the most appropriate and accepted term.