Niddah, Chapter 5, Mishnah 7. This chapter walks through the stages of a person's growing up and the halachic consequences that attach to each age. Our mishnah turns to a very practical question: from what age does a child's vow, or a child's declaration making property hekdesh to the Beis Hamikdash, actually take effect?
The girl: the eleventh year and the twelfth
The mishnah begins with the daughter. A girl who is "bas achas esreh shanah" plus a single day ("veyom echad") occupies an in-between status. "Nedareha nivdakin": her vows are neither confirmed on the spot nor dismissed on the spot, but tested. Beis din probes her understanding, asking whether she is aware of Whom a neder invokes and whether she can say what obligation she has taken upon herself. If her answers show genuine awareness, what she said stands.
When a full twelve years have gone by along with an additional day ("bas shtem esreh shanah" and "veyom echad"), the mishnah rules "nedareha kayamin." At that point there is nothing left to clarify; her nedarim are binding on their own strength.
"Uvodkin kol shtem esreh." The examination is not a one time procedure. Throughout the entire year in which she is eleven, every vow she makes is examined afresh. We do not question her once, at her first vow of the year, and then coast on that result for the remaining months. The reason is simple and rather sensitive: a girl who did not yet grasp in whose Name she was vowing at the start of the year may very well grasp it several months later, as she matures. Each vow therefore gets its own inquiry.
The boy: the same rule, two years later
For a son the mishnah sets out exactly the same framework, pushed forward by twelve months. "Ben shtem esreh shanah" together with a day ("veyom echad"): "nedarav nivdakin," meaning his vows fall into the category that requires investigation. From "ben shloshah asar shanah" and a day, the mishnah says "nedarav kayamin," and no probing is called for.
"Uvodkin kol shloshah asar." Here too, the questioning is repeated throughout the entire year of his being twelve. We do not lean on an earlier examination that came out negative and assume he still lacks understanding. Comprehension arrives with growth, and a boy who could not answer in the fall may answer well in the spring, so each new vow calls for a new inquiry.
Before and after
The mishnah now takes up the years on either side of this window. "Kodem lazman hazeh": at any point earlier than the ages just given (a girl below eleven and a day, a boy below twelve and a day), nothing the child asserts carries halachic weight. The mishnah presses the point: "af al pi she'amru" (even if the children announce) "yod'im anu," insisting that they are perfectly aware of Whom their neder invoked and to Whom the object was consecrated ("leshem mi hikdashnu"), the ruling remains "ein nidreihem neder ve'ein hekdeshan hekdesh." No vow has come into being, and no item has become sanctified property of the Beis Hamikdash.
"Le'achar hazman hazeh": from the later milestone onward (a girl of twelve years and a day, her bas mitzvah, and a boy of thirteen years and a day, his bar mitzvah), the ruling reverses. Even should they protest "ein anu yod'im," pleading ignorance of Whom their neder invoked and to what Name the object was consecrated, the mishnah concludes "nidran neder vehekdeshan hekdesh." The vow is fully in force, and the consecration is real.
The picture that emerges is of three periods: a stretch of years in which nothing the child says can create a vow, a single transitional year in which each and every vow is tested by questioning, and then adulthood, in which the words themselves do the work regardless of what the person claims about his own understanding.