1) TESTIFYING ABOUT SOMETHING ONE SAW WHILE HE WAS A MINOR
QUESTION: The Mishnah lists the topics about which a person may testify when he is an adult even though he witnessed them when he was a Katan. If a person is trusted to testify about these things that he witnessed when he was a Katan, why does he have to be an adult to testify about them? He should be able to testify about them while he is a Katan!
(a) The RITVA and TOSFOS RID explain that the Mishnah is teaching that although normally a witness must be a valid witness both at the time that he sees the event and at the time that he testifies about it, in the cases mentioned in the Mishnah (such as Kiyum Shtaros) it suffices that the witness be a valid witness only at the time he testifies, because these topics are all mid'Rabanan. (See also RAMBAM, Hilchos Edus 14:2.)
The Tosfos Rid adds that his testimony is accepted only when he is an adult because once it is clear that he, as an adult, personally relies on what he saw as a child, it may be assumed that he indeed carefully examined it as a child and recognizes it well. This explains why the Rabanan were not lenient to accept his testimony when he says it as a Katan.
(b) The RAMBAN (in MILCHAMOS, 8b of the pages of the Rif) explains that testimony about the veracity of a Shtar differs from other types of testimony. In other cases, the witness is testifying about what he previously saw. If he was not a valid witness at the time that he saw the event, Beis Din cannot accept his testimony even though he is a valid witness when he testifies. In contrast, when he testifies about the veracity of a Shtar, the witness is testifying that he presently recognizes the signature, and it does not matter when he first saw the signature since his testimony involves only what he sees now.
The Gemara states that he is believed to testify about what he saw when he was a Katan only because Kiyum Shtaros is mid'Rabanan. According to the Ramban, it is unclear why this reason is necessary. Since he is a valid witness who testifies about what he sees now, even if Kiyum Shtaros would be mid'Oraisa he should be believed.
It must be that although the witness himself is a valid witness, there is another problem with testimony that a witness says based on what he saw as a Katan. Since a Katan does not carefully examine what he sees, even though he now testifies as an adult on what sees now (the signatures of witnesses), his testimony cannot be accepted for any d'Oraisa purpose because it is based on what he saw without careful examination when he was a Katan. This idea is implied by the Gemara when it suggests that a witness is believed to testify about something he saw when he was a Katan only when he testifies about his own father's or Rebbi's signature, since he indeed is more careful to study their signatures and to recognize them than other signatures.
2) A MINOR'S TESTIMONY ABOUT AN ISUR D'RABANAN
QUESTION: The Mishnah implies that a person may not testify when he is an adult for something he saw when he was a Katan, even for an Isur that is only d'Rabanan. TOSFOS asks that the Gemara in Pesachim (4b) contradicts this when it says that a Katan is believed when he says that Bedikas Chametz (which is d'Rabanan) was performed in a certain house. How are the two Gemaras to be reconciled?
(a) TOSFOS answers that the Rabanan permitted a Katan to testify in the case of Bedikas Chametz because it is in his ability to search the house for Chametz himself. In the cases of the Mishnah, however, the subject of his testimony (such as Kehunah or Techum Shabbos) is not within his control, it is not "b'Yado."
(b) The RASHBA adds that in the case of Bedikas Chametz, the Katan says that he himself searched for Chametz. He is believed when he says something that he did himself because he is more careful about it and takes it more seriously.
(c) The RASHBA answers further that Bedikas Chametz is a procedure that is performed every year and it involves everyone. Therefore, a child knows what he is talking about when he testifies that Bedikas Chametz was done (in contrast to testimony about signatures and the like). (See also REMA, end of YD 127.)
3) A MINOR'S TESTIMONY THAT A FAMILY IS "PESULAH"
QUESTION: The Gemara cites a Tosefta that states that a child is believed to testify that his father said a certain family is Kesherah and that another family is Pesulah.
Why is the child's testimony accepted? The Mishnah states that a Katan is believed to testify about certain things only after he becomes an adult!
(a) RASHI and the RASHBA explain that the Tosefta also refers to when the Katan testifies after he becomes an adult. (The words of the Tosefta, "Ne'eman Tinok...," are not consistent with this explanation.)
(b) The RITVA and RAN in the name of the RA'AVAD explain that the Tosefta indeed refers to a case in which the Katan testifies when he is a Katan. Beis Din does not accept his testimony absolutely, but merely takes his words into consideration.
(c) The RITVA explains further that the Tosefta refers to a case in which it is already known that one of the two families mentioned by the Katan is Pesulah, but it is not known which family. The Katan's testimony merely clarifies to Beis Din which one is Pesulah. Since he is merely clarifying a situation which is already known, Beis Din may accept what he says. However, he is not believed when he testifies that one of two families is Pesulah when it was not known heretofore that either family was Pesulah.
The Ran suggests a similar explanation but limits the trustworthiness of the Katan. The Katan is believed only for those whom he says are Kasher, but not for those whom he says are Pasul.