1,918. A Minor Yavam
Yibum v’Chalitzah 5:18
If a minor yavam over the age of nine years and a day has relations with his yevama*, it is the halachic equivalence of maamar given by an adult. Accordingly, marriage is not wholly effected. If a minor yavam over nine years and a day gives maamar without it being preceded by any other action, it is effective and prohibits the yevama to any other adult brothers. However, if he gives maamar after one of his adult brothers, it is an insignificant act. A get or chalitzah from a minor is always a meaningless act regardless of whether it precedes or follows actions taken by adult brothers.
Yibum v’Chalitzah 5:19
Pursuant to the previous halacha, if a minor yavam who is over the age of nine years and a day engages in relations with his yevama* or gives her maamar without any other action preceding it, he disqualifies her from yibum with any other brothers. However, if an adult brother gave her maamar, and then the minor of nine years and a day gave maamar to her or to another of the widows, his action is of no significance and does not prohibit the yevama to his brother. However, if the minor of nine years and a day engages in intimacy with her or with one of the other widows, he does prohibit her to his brother. This is the same as when two adult brothers each give maamar in succession, as has been discussed.
*The intention of the halacha is not to recommend such a thing, merely to address the halachic impact should it occur.