R. Joseph said: Come and hear! A maiden aged three years and a day may be acquired in marriage by coition, and if her deceased husband's brother cohabits with her, she becomes his.

MISHNAH. A girl of the age of three years and one day may be betrothed by intercourse; if the yabam had intercourse with her, he acquires her thereby; the guilt of adultery may be incurred through her, and she causes uncleanness to the man who had intercourse with her so that he in turn conveys uncleanness to that upon which he lies, as to a garment which has lain upon [A Zab]. If she was married to a priest, she may eat Terumah. If any of the ineligible persons cohabited with her he disqualifies her from the priesthood. If any of the forbidden degrees enumerated in the Torah cohabited with her he is to be executed on her account, but she is exempt [from the penalty]. If one was younger than this age intercourse with her is like putting a finger in the eye.

GEMARA. Our Rabbis taught: A girl of the age of three years may be betrothed by intercourse; so R. Meir. But the Sages say: Only one who is three years and one day old. What is the practical difference between them? — The school of R Jannai replied: The practical difference between them is the day preceding the first day of the fourth year. R. Johanan, however, replied: The practical difference between them is the rule that thirty days of a year are counted as the full year.

An objection was raised: A girl of the age of three years and even one of the age of two years and one day may be betrothed by intercourse; so R. Meir. But the Sages say: Only one who is three years and one day old. 

Moving on to Folio 45a:

Now, all is well according to R. Johanan, for just as there is a Tanna who holds that one day of a year is counted as a year so there may also be a Tanna who holds that thirty days of a year are counted as a full year; but, according to R. Jannai, does not this present a difficulty? — This is a difficulty.

If one was younger than this age, intercourse with her is like putting a finger in the eye. It was asked, Do the features of virginity disappear and reappear again or is it possible that they cannot be completely destroyed until after the third year of her age? In what practical respect could this matter? — In one, for instance, where her husband had intercourse with her before the age of three and found blood, and when he had intercourse after the age of three he found no blood. If you grant that they disappear and reappear again [it might well be assumed] that there 'was not sufficient time for their reappearance, but if you maintain that they cannot be destroyed until after the age of three years it would be obvious that a stranger cohabited with her. Now what is your decision? — R. Hiyya son of R. Ika demurred: But who can tell us that a wound inflicted within the three years is not healed forthwith, seeing it is possible that it is immediately healed and it would thus be obvious that a stranger had cohabited with her? Rather the practical difference is the case, for instance, where her husband had intercourse with her while she was under three years of age and found blood and when he had intercourse after the age of three he also found blood. If you grant that the features disappear and reappear again the blood might well be treated as that of virginity, but if you maintain that they cannot be destroyed until after the age of three years, that must be the blood of menstruation. Now what is your decision? — R. Hisda replied, Come and hear: If one was younger than this age, intercourse with her is like putting a finger in the eye; what need was there to state, 'like putting a finger in the eye' instead of merely saying: if one was younger than this age, intercourse with her is of no consequence'? Does not this then teach us that as the eye tears and tears again so do the features of virginity disappear and reappear again.

Our Rabbis taught: It is related of Justinia the daughter of 'Aseverus son of Antonius that she once appeared before Rabbi 'Master', she said to him, 'at what age may a woman marry?'. 'At the age of three years and one day', he told her. 'And at what age is she capable of conception?' 'At the age of twelve years and one day', he replied. 'I', she said to him, 'married at the age of six and bore a child at the age of seven; alas for the three years that I have lost at my father's house'. But can a woman conceive at the age of six years? Did not R. Bibi recite in the presence of R. Nahman: Three classes of woman may use an absorbent in their marital intercourse: A minor, and an expectant and a nursing mother. The minor, because otherwise she might become pregnant and die. An expectant mother, because otherwise she might cause her foetus to degenerate into a sandal. A nursing mother, because otherwise she might have to wean her child prematurely, and this would result in his death. And what is the age of such a 'minor'? From the age of eleven years and one day to the age of twelve years and one day. One who is under or over this age must carry on her marital intercourse in a normal manner; so R. Meir. But the Sages ruled: The one as well as the other carries on her marital intercourse in a normal manner and mercy will be vouchsafed from heaven, for it is said in Scripture, The Lord preserveth the simple? — If you wish I might reply: Whose flesh is as the flesh of asses. And if you prefer I might reply: Whose mouth speaketh falsehood, and their right hand is a right hand of lying.

Our Rabbis taught: A story is told of a certain woman who came before R. Akiba and said to him, 'Master, intercourse has been forced upon me when I was under three years of age; what is my position towards the priesthood?' 'You are fit for the priesthood', he replied. 'Master', she continued, 'I will give you a comparison; to what may the incident be compared? To a babe whose finger was submerged in honey. The first time and the second time he cries about it, but the third time he sucks it'. 'If so', he replied, 'you are unfit for the priesthood'. Observing that the students were looking at each other, he said to them, 'Why do you find the ruling difficult?' 'Because', they replied, 'as all the Torah is a tradition that was handed to Moses at Sinai so is the law that a girl under the age of three years is fit for the priesthood one that was handed to Moses at Sinai'. R. Akiba too made his statement only for the purpose of exercising the wits of the students.

MISHNAH.  If a boy of the age of nine years and one day cohabited with his childless brother's widow, he acquires her thereby, but he cannot divorce her until he attains his majority. He contracts uncleanness through intercourse with a menstruant and he in turn conveys the same degree of uncleanness to that upon which he lies as [does A Zab] to that which has lain upon him. He disqualifies a woman from the priesthood, but cannot confer upon one the right to eat Terumah. He renders a beast invalid for the altar, and it is stoned on his account. If he had intercourse with any of the forbidden degrees that are enumerated in the Torah, she is to be executed on his account, though he is exempt from punishment.

GEMARA. But when he attains his majority, is a divorce alone sufficient? Was it not taught: The cohabitation of a boy of nine years of age was given the same validity as that of a ma'amar by an adult; as a ma'amar by an adult requires a divorce in respect of his ma'amar and halizah in respect of his marital bond so does the cohabitation of a boy of nine years of age require a divorce in respect of his ma'amar and halizah in respect of his marital bond? — Rab replied: It is this that was meant: