Yevamos, Chapter Ten, Mishnah 8. Before us is a further discussion of the law of a boy nine years and one day old who had relations with his yevamah, and of the status of that act.
The Mishnah opens: "Ben teisha shanim v'yom echad sheba al yevimto" - a boy who has reached the age of nine years and one day: his act of relations counts as a valid act, but he has not yet reached adulthood. The Sages therefore treated his act of relations as having the status of a ma'amar - the conditional acquisition made by an adult.
"V'achar kach ba tzaratah":
After this act, the boy had relations with her tzarah - the second wife of that same brother - something he is not permitted to do, since once he has performed yibum with one of the co-wives he can no longer perform yibum with the second; nevertheless, he did so in practice.
Had he been an adult, once he had relations with one of the yevamos she would have become his full wife, and anything he did with her co-wife would be irrelevant. Here, however, since his act of relations is only like a ma'amar, the zikah remains in place and can still be acted upon. Therefore, once he had relations with the co-wife - "pasal l'atzmo" - he disqualified the first one by his own act, for the zikah remained and he then had relations with the second, so that both of them are now forbidden to him.
The view of Rabbi Shimon:
Rabbi Shimon disagrees and says: "lo pasal" - he did not disqualify. His approach is based on what he said in the previous Mishnah: he does not view the relations of a minor as a partial acquisition or a partial result, but rather as a doubt: either it is fully effective, exactly like the relations of an adult, or it is not effective at all. Accordingly, the relations with the co-wife have no effect whatsoever: if the relations of a nine year old are effective, then the first is his wife and the second is nothing; and if they are not effective, then the second act accomplished nothing either. Either way, the relations with the second woman do not change the status at all.
The second case: he had relations with his yevamah and died:
"Ben teisha shanim v'yom echad sheba al yevimto" - and then he died. Now the woman falls to yibum before the third brother, and he stands before a woman who has a zikah from the first husband and a partial zikah from the second - for the nine year old had relations with her, and his act is like a ma'amar. This case resembles what we learned in the third chapter: one who made a ma'amar with his yevamah and died leaves the woman bound by two zikos. And we learned from the verse - and some understand that the matter is rabbinic - that a woman upon whom there are two zikos is not taken in yibum; rather, chalitzah is performed for her. So too here: if he had relations with her and died, she performs chalitzah and does not undergo yibum.
A minor who betrothed a woman and died:
An entirely different case is where there is no yibum involved at all: a minor who betrothed a woman and died before reaching the age of thirteen - the woman is exempt, both from yibum and from chalitzah, since the betrothal of a minor does not take effect, and she is not his wife at all.
The only case in which we attribute any force to the relations of a minor is in yibum, because there he acts by virtue of the zikah that already exists between him and the yevamah. He operates within a bond that existed beforehand, and therefore the act of relations has a certain effect - whether by Torah law or by rabbinic law. But in ordinary betrothal his actions have no force at all, and if he died she is entirely exempt from yibum and from chalitzah.
In summary: In this Mishnah we learned that the relations of a boy nine years and one day old with his yevamah have the status of a ma'amar only, and therefore one who afterward has relations with her co-wife has disqualified both of them by his own act; while according to Rabbi Shimon, who views the relations of a minor as a doubt and not as a partial acquisition, he did not disqualify at all. We also learned that if he had relations with his yevamah and died, the woman is left bound by two zikos and performs chalitzah rather than undergoing yibum; whereas a minor who betrothed a woman and died - his betrothal does not take effect and the woman is exempt from yibum and from chalitzah.