Yevamot, Chapter Ten, Mishnah Six
This mishnah discusses the ramifications of a boy, nine years and one day old, performing an act of yibbum, divorce, halitzah or maamar. The mishnah teaches that a boy of this age is capable of having sex which is a purely physical act. However, he does not have the requisite knowledge (daat) to give a get, perform maamar or halitzah. One other issue which we must keep in mind while learning this mishnah is that once a woman awaiting yibbum (a shomeret yavam) has had one of these acts performed with her by one of the brothers, she may no longer marry any of the other brothers of her dead husband.
We should note that in the time of the Mishnah, not only was marriage considered possible at a very young age, but concepts of childhood were different. Our perception of childhood as extending well into ones teens and our assumption that children deserve protection during these years and are not to be treated as adults, are, to a large extent, more modern perceptions. The mishnah, as we shall see, is therefore not squeamish about discussing sex and children. Today, in our society children are (or should be) distanced from sex.
1) A boy of the age of nine years and one day disqualifies [his sister-in-law for marriage] with his brothers, and his brothers disqualify her for him, but while he disqualifies her from the outset only, the brothers disqualify her from the outset and at the end.
2) How is this so? A boy of the age of nine years and one day who had intercourse with his sister-in-law disqualifies her [for marriage] with his brothers;
3) The brothers disqualify her [for marriage with him] whether they had intercourse with her, did maamar, gave her a get or submitted to her halitzah.
Section one: The first section of the mishnah is a brief, memorable way of stating that which will be explained in section two and in the following mishnayoth. I will explain the concepts as they are fleshed out in the mishnah.
Section two: If a boy of nine years and one day has intercourse with a shomeret yavam, that is his dead brothers wife, the other, older brothers may no longer have yibbum/intercourse with her. This means that his act of intercourse counts sufficiently that she is tied to him, and no longer liable for yibbum or halitzah to the other brothers. Since she is no longer liable for yibbum or halitzah, they are forbidden to have intercourse with her.
Section three: In contrast to the nine year old boy, the older brothers disqualify the shomeret yavam from having relations with any of the other brothers in four different ways: intercourse (yibbum), maamar (betrothal), divorce (get) and halitzah. Even though maamar and a get are only of rabbinic origin for the shomeret yavam, and she still requires halitzah before she may remarry, they are sufficient to disqualify her from the other brothers.
The difference between the older brothers and the nine year old brother is that for the older brothers there are legal consequences to their performance of acts that require knowledge, such as maamar, divorce, or halitzah, whereas for the nine year old, there are legal consequence only to physical acts such as intercourse.