(Photo of Johnny, a guevedoce, via the Daily Telegraph)
They're called the guevedoces, which translates into English as "penis at twelve." The isolated village of Salinas in the Dominican Republic is the center of an extremely rare genetic disorder. About 1 in 90 children born there have it. When these guevedoces are born, they appear to be anatomically female--or at least lacking external male genitalia. Then, sometime between the age of 7 and 12, hormonal changes cause a functional penis to grow. The Daily Telegraph explains why this happens:
But some male babies are missing the enzyme 5-α-reductase which triggers the hormone surge, so they appear to be born female with no testes and what appears to be a vagina. It is not until puberty, when another huge surge of testosterone is produced, that the male reproductive organs emerge. What should have happened in the womb happens around 12 years later. Their voices deepen and they finally grow a penis.
Dr. Julianne Imperato, an endocrinologist at Cornell University, first documented the phenomenon in the 1970s:
When Dr Imperato investigated the Guavadoces she discovered the reason they don’t have male genitalia at birth is because they are deficient in an enzyme called 5-α-reductase, which normally converts testosterone into dihydro-testosterone. [...]
Around one in 90 children in Salinas are guevedoces and although they resemble sexually normal males, subtle differences do still exist in adulthood. Most have decreased amounts of facial hair and smaller prostate glands relative to the average male.