According to Today I Found Out, the youngest girl to ever have her menstrual cycle was Lina Medina at the age of 2.5. She was only 5 when she gave birth to her first child. This was possible due to a condition called precocious puberty.
Today I Found Out says that precocious puberty is an early onset of sexual development. This caused Medina to have her first menstrual cycle at the age of 2.5 and experience breast development at the age of 4. By 5, she had fully working sexual organs. The circumstances of her impregnation were never made known, but an investigation observed that some of the festivities in her village ended in orgies, which sometimes included rape.