הלכה אויף שמות 21:7

A girl from the day of her birth until twelve complete years of age is called a minor and is [also] called a child. And even if she has grown several hairs within this time, they are merely like a mole. But if she has grown two hairs below on her body in places known for hair growth, and she is [at least] twelve years and one day old or more, she is called a young woman.

The growth of two hairs during this time is called the “lower sign”. After the appearance of the lower sign, she is called a young woman for the next six complete months. And from the day beginning the conclusion of [these] six months and afterwards, she is called a woman of age. The length of time between becoming a young woman and a woman of age is only six months.