1,434. Lower But Not Upper, Upper But Not Lower

Hilchos Ishus 2:8

If a girl exhibits one – or even all eight – of the upper signs of maturity before she reaches 12 years of age, it is of no consequence. Once she reaches the age of 12 years and one day old, if she exhibits the lower sign of maturity (i.e., two public hairs), then the presence or absence of the upper signs makes no difference. If she doesn’t exhibit the lower sign but she does exhibit one of the upper signs, there is a question as to whether she is a minor or a naarah, so we take the more stringent position in any halachic matter. If she exhibits all of the upper signs, she is definitely considered an adult because it would be impossible to exhibit all of these signs otherwise. We assume that she must have grown the requisite pubic hairs and then subsequently lost them.

Hilchos Ishus 2:9

If a woman gives birth after reaching the age of 12, she is considered an adult even if she didn’t exhibit either the upper or lower signs of maturity. Giving birth is itself a sign of maturity.