Yevamot, Chapter Thirteen, Mishnah Twelve
The first half of this mishnah discusses the case of a minor yavam (up until now the mishnah has concentrated mostly on the minor yevamah).
The second half of the mishnah discusses the situation of a woman who claims that her yavam has not had yibbum with her and she wants halitzah.
1) If a yavam who was a minor had intercourse with a yevamah who was a minor, they should be brought up together.
2) If he had intercourse with a yevamah who was of age, she should bring him up until he is of age.
3) If a yevamah declared within thirty days [after yibbum], he has not had intercourse with me, they force him to perform halitzah;
4) [If her declaration was made] after thirty days, they request that he perform halitzah.
5) If he admits [that he did not have intercourse with her], they force him to perform halitzah.
Section one: If a minor yavam who is legally capable of having sexual intercourse (over nine years old) has intercourse with a minor yevamah, he cannot divorce her because he cannot give a get until he reaches majority age. Therefore, they must remain together until they reach majority age, at which point they can decide whether to remain married or separate through divorce.
Section two: So too if he has intercourse with a yevamah who is of majority age; she must wait with him until he is of majority age.
Section three: The second part of the mishnah deals with a case where the yevamah claims that the yavam did not have intercourse with her, and she wants to have halitzah. If she makes this claim within thirty days, she is believed, and the court forces the husband to perform halitzah. This is because we assume that for thirty days the yavam may actually not have had yibbum with her.
Section four: If she makes this declaration after thirty days, she is no longer automatically believed, for there is an assumption that the yavam would not have waited for more than thirty days to have yibbum. In this case, all the court can do is request that he perform halitzah for her.
Section five: If he admits that he did not have yibbum with her, he is forced to have halitzah, even after twelve months. He has a responsibility to her and cannot just leave her hanging, legally bound to him and without the possibility of marrying another.