Today in 1939 Lina Medina a Peruvian 5 year old become the youngest mother in history. Wired has the story, but it seems that she had an extreme case of the extremely rare precocious puberty. The story was only accepted as true after a photo (on the Wikipedia story) and X-rays of the case were revealed.
The child lived until 40 (died of bone marrow disease) and Lina is still alive. Bizarre story.