We already mentioned earlier that when a boy reaches the age of nine years and one day, "bi'aso bi'ah" - his act of intercourse counts as intercourse. From that point on it is regarded as a physical act of marital relations, and various legal consequences follow from it.
The status of a nine year old boy with regard to a yevamah:
The Mishnah rules that a nine year old boy disqualifies the yevamah for the remaining brothers, and likewise the remaining brothers disqualify her for him. That is, if he had relations with her, or performed ma'amar with her by giving her money or a document, she becomes forbidden to the other brothers; and conversely, if the brothers did so, she becomes forbidden to him.
The difference between the nine year old and the other brothers:
The nine year old: he disqualifies only at the outset, that is, only if he is the first one to perform any act. If the older brother performed ma'amar with her, and afterward the nine year old performed ma'amar with her, his ma'amar is nothing and has no validity.
The other brothers: they disqualify both at the outset and at the end. Whether their ma'amar came first or the ma'amar of the nine year old came first, they still have the power to disqualify her from entering yibum with him.
These words are not stated explicitly in the Mishnah, but the Gemara teaches that they must be added. In what case does this apply? When ma'amar is the instrument being used. But intercourse, even in the case of the minor, is effective even at the end, and has the power to disqualify her for the other brothers.
Ma'amar versus intercourse, in practice:
A nine year old who had relations with the yevamah: he disqualifies her from entering yibum with the other brothers.
The other brothers: if they had relations with her, or performed ma'amar with her, or gave her a get, or performed chalitzah with her, even after the intercourse or the ma'amar of the nine year old, they disqualify her for him.
To summarize: in the case of the nine year old, only intercourse disqualifies after the fact, that is, after the brothers have already performed ma'amar or some other act with her. Intercourse is the exception: if they performed ma'amar with her and afterward the nine year old had relations with her, he disqualified her. This is not so, however, if the nine year old only performed ma'amar with her.