Niddah 5:8-9

Niddah 5:8

The signs of female maturity: Rabbi Yosi HaGlili says when the wrinkle appears under the bust; Rabbi Akiva says when the breasts slope; Ben Azzai says when the areolas darken; Rabbi Yosi says when, after putting a hand on the nipple, it sinks in and returns slowly.

Niddah 5:9

If a 20-year-old woman hasn’t produced two pubic hairs, she should bring proof of her age and that she’s an ailonis (one who doesn’t mature). Such a woman performs neither yibum nor chalitzah. If a 20-year-old man hasn’t produced two pubic hairs, he should bring proof of his age and that he’s congenitally infertile. He also performs neither yibum nor chalitzah. This is the opinion of Beis Hillel; Beis Shammai say an 18-year-old woman and an 18-year-old man. Rabbi Eliezer says the halacha regarding a man is like Beis Hillel but a woman is like Beis Shammai because women mature earlier than men.