While on the topic of shlichus – why is it that a katan cannot be a shaliach? I count two or possibly three reasons, depending on how finely you want to split hairs. Rashi in Bava Metziya (71b) points to the derivation of shlichus from the parsha of hafrashas terumah. Since a katan cannot be mafrish terumah or make nidrei hekdesh, he is excluded from shlichus. Rashi in Kiddushin (42a) points to the derivation of shlichus from the parsha of gittin where the Torah uses the term “ish” (“ki yikach ish”), which excludes a katan. Now we get to the hairsplitting part. The way I read Rashi is that the exclusion is based on the gezeiras hakasuv that specifies “ish”. The last Ketzos in Hil. Shlichus, however, explains that a katan is excluded because a katan cannot marry or divorce. What difference does it make if you read it as an explicit gezeiras hakasuv or an implicit sevara? If it is a gezeiras hakasuv I’m not sure you need to be bothered by the Ketzos’ problem of how to exclude a ktana who is capable of getting married provided her father accepts kiddushin on her behalf.
The Ketzos explains that the two reasons offered by Rashi may depend on the issue of whether the halacha of “mufla hasamuch l’ish” is a din d’oraysa or not (as discussed in a recent daf yomi). If “mufla hasamuch l’ish” is a din d’oraysa, it means a child of 12, who is technically a katan, is still capable of being mafrish terumah or making nidrei hekdesh. Therefore, we cannot derive the exclusion of a katan from shlichus from the parsha of terumah. Instead, we have a source from the parsha of gittin.
This same issue may explain a difficult Rambam. Whatever the reason a katan/ktana cannot be a shliach, the same logic would also disqualify him/her from being a meshaleyach as well. The Rambam (Gittin 6:9) writes that a ketana cannot appoint a shliach kabbalah to receive her get because witnesses must testify that the shliach was properly appointed, and witnesses cannot testify as to the competence of a ketana who lacks da’as to make such an appointment. The Ra’avad offers a far simpler reason – the parsha of terumah teaches us that a katan/ketana is excluded from shlichus. Why does the Rambam need to re-invent the halachic wheel and suggest a new sevara?
The Ketzos explains that the Ra’avad must hold that that a mufla samuch l'ish cannot be mafrish terumah and therefore the parsha of terumah serves as a perfect source for the exclusion of a katan (like Rashi in Bava Metziya). Rambam disagrees and holds that a katan (who is mufla samuch l’ish) can be mafrish terumah and therefore suggests a sevara as a source.
I would be happy if I could find or work out another approach to the Rambam which still accepts the the parsha of terumah as the source for the exclusion of a katan but requires the additional sevara to explain some unique aspect of shlichus kabbalah. Don’t have this worked out yet, so consider it food for thought.