Yevamot, Chapter Ten, Mishnah Seven
This mishnah explains the status of an act of sexual intercourse done by a boy who has reached nine years and one day old, but is not yet of majority age.
If a boy of the age of nine years and one day had intercourse with his yevamah and then his brother who was of the age of nine years and one day had intercourse with her, [the latter] disqualifies her for [the former].
Rabbi Shimon says: he does not render her unfit.
As we learned yesterday, the sexual intercourse of a boy nine years and one day old has legal ramifications. In our case of yibbum, if a boy who has reached this age has relations with his yevamah, he disqualifies her from being permitted to have yibbum with another one of the brothers. However, this act of sexual intercourse is not sufficient to completely make her his wife. The Talmud states that it is not a complete acquisition, as it would be if he was of majority age. Therefore, when another brother, who has also reached this age, has intercourse with her, he causes her prohibited to the first brother. Since she has now had quasi-yibbum with two brothers (akin to maamar with two brothers, see above 5:1), she is forbidden to both.
Rabbi Shimon holds that it is doubtful whether sexual intercourse with a boy nine years and one day old can acquire a woman as a wife. In our case, if it does, then she is the wife of the first brother, and the intercourse with the second brother does not disqualify her to the first. If such intercourse does not acquire her as a wife, then neither does the intercourse with the second brother, and she is not disqualified to the first brother. Should she wait around as a shomeret yavam until he is of majority age, he may then have yibbum with her.
To summarize, the first opinion holds that an act of intercourse by a boy nine years and one day, certainly acquires a woman as a wife, but the acquisition is not complete. In contrast, Rabbi Shimon holds that it is uncertain whether this act of intercourse acquires. However, if it does acquire, it acquires fully.