The sages ruled that one who has sexual intercourse with an infant under the age of three is not considered to have had sexual intercourse. The Halachic implication is that if there is incest with a child under three (for example, a father who has sexual relations with his under three year old daughter) it is not considered incest and the father is exempt from punishment. The sages explained their determination by saying that having sexual relations with a girl under the age of three is like sticking a finger in a person’s eye: she will remain a virgin and so the sexual intercourse is without significance. The scholars discussed how this infant remains a virgin after sexual relations: is it because the hymen of an infant who has had sex is torn and then heals, or is it because the hymen does not tear in the first place? What does it matter what the reason is? Answer: These explanations have Halachic implications. Is the infant considered licentious and thus forbidden to a cohen? A man has sex with an infant under the age of three and she bleeds in the wake of this intercourse. After she grows and passes the age of three a cohen has intercourse with her and she does not bleed in the wake of intercourse. If we follow the first opinion, that the hymen tears and heals, perhaps she did not bleed when having intercourse with the cohen because not enough time had passed for her hymen to heal. But if we follow the second school of thought, that her hymen doesn’t tear at all, there is a suspicion that she had sexual intercourse with another man after she turned three, and that is why the sexual relations with the cohen did not cause bleeding; her hymen was torn after she turned three. The girl is then considered licentious and forbidden to a cohen. One of the sages rejected this conclusion, this Halachic implication. In his opinion, based on both schools of thought we must suspect she had sexual relations again after she turned three, and that is why she is considered licentious and forbidden to a cohen. In his opinion there is a different conclusion, one called for in the case of a man who has sexual intercourse with an infant under the age of three and blood is shed as a result of the intercourse. If we suppose that under the age of three her hymen is not torn, then the blood is menstrual blood and she is impure, but if we suppose that her hymen tears and heals, then she is not impure with niddah impurity and the blood which is shed is hymeneal blood. (The Talmud rules that the hymen of an infant under the age of three tears and heals.)
(Babylonian Talmud, Tractate Niddah 45a)