In msh210's answer to this question of mine, he stated that:
women...who physically could be [intimate with men], which is those who had reached their third birthday.
In comments, he mentioned that:
age three is the age at which a girl is halachically considered to be physically able to have intercourse.
Could you explain why and where this comes from? Keep in mind I am not Jewish, so if it specific to some non-OT writings, please explain those as well.