Once the gemara establishes that the mishna from Migila is in accordance with R' Yehuda, and that the sefa of that mishna (in migila) asserts that a katan that's reached the age of chinuch can read and be yotzei others le'chatchila, where does that leave the riesha? Can a cheresh ha'midaber (has to be -- how else could he even potentially read the migila) read and, be'dieved, be yotzei others as well? (A little bit similar to the trumah case -- ideally a cheresh ha'midaber shouldn't take trumah, but if he does, it works).
I also looked up the case in the Rambam, and he (seems to) leave it ambiguous -- he doesn't address the case of a cheresh!
Josh Chiel, Cleveland, USA
Josh, Baruch she'Kivanta! The Rishonim discuss your question!
1) The Rashba writes that b'Di'eved a Cheresh ha'Medaber can be Motzi others with his reading of the Megilah. This is learned from the Gemara (end of 15a) which asks, "Who is the Tana who maintains that a Cheresh cannot be Motzi even b'Di'eved? It is Rebbi Yosi." This suggests that it is only Rebbi Yosi who disqualifies the Cheresh even b'Di'eved, while Rebbi Yehudah approves of him b'Di'eved. The reason why Rebbi Yehudah mentioned only the minor in the Seifa is because he wanted to make a distinction between a Katan and a Cheresh; namely, that a Katan may read l'Chatchilah while a Cheresh may read only b'Di'eved.
2) The Rashba also cites the Ra'avad who maintains that Rebbi Yehudah permitted the Cheresh even l'Chatchilah.
3) The text in the Frankel edition of the Rambam (Hilchos Megilah 1:2) is, "If the reader was a minor or a Shoteh, the person who heard him reading the Megilah was not Yotzei." According to this text, the Rambam's omission of Cheresh implies that the Cheresh may be Motzi others. This text fits better with our Gemara, since our Gemara rules against Rebbi Yosi, implying that the Halachah should be that the Cheresh may be Motzi others. The Mishnah Berurah (689:5) writes that several Acharonim maintain that even a Cheresh who cannot hear at all still may be Motzi others. The Sha'ar ha'Tziyun #9 writes that one of these Acharonim is the Gra. If we look at the Bi'ur ha'Gra (689:2) we see that he writes that in his text of the Rambam, Cheresh is not stated. This means that the Vilna Gaon maintains that according to the Rambam a Cheresh may be Motzi others, b'Di'eved.