67:9 When it comes to keeping mitzvos, a boy is not obligated until he reaches the age of 13 and a girl is not obligated until she reaches the age of 12. When it comes to vows and oaths, they become obligated a year earlier. Therefore, a boy aged 12 years and one day, or a girl aged 11 years and one day, are obligated. This is true even if they do not yet show the physical signs of maturity, so long as they understand to Whom they are making vows or oaths. Younger than that, their statements do not constitute vows or oaths even if they understand to Whom those statements are addressed. If a younger child does make such a statement, even though it is not effective as a vow or an oath, he should be reprimanded so that he does not get in the habit of making vows and oaths. If the child vowed in a small matter that causes no hardship, he should be compelled to observe that which he said.
67:10 A father can annul vows made by his unmarried daughter until she reaches the age of twelve and a half. A husband can annul vows made by his wife. Vows are annulled by saying three times that they are annulled or nullified or any other terminology that makes it clear that the vow has been negated as of its very beginning. This may be done even if the woman is not present. A father or a husband cannot annul a vow using the text that is used by the panel of three. Also, a father or husband can only annul a vow on the day he learned of it. If he heard about it at the beginning of the night, he can annul it that whole night and all the following day (because the Jewish day begins at nightfall). If he heard about it right before the stars came out, he can only annul it until the stars appear; after that, he can no longer annul the vow. One should not say, “Your vow is nullified” on Shabbos the way that one does on weekdays. Rather, he should annul the vow mentally and then instruct his wife or daughter to do something that goes counter to the vow. (If the father or husband does not know about the vow and it does not involve some Shabbos need, it is best not to inform him about it on Shabbos – Mishnah Brurah 341:3.) If the father or husband initially expressed approval of the vow, either explicitly or implicitly, or even if he just thought his approval internally, he is no longer able to annul the vow.