During the weeks between Pesach and Shavuot, we count 49 days until kabbalat ha’Torah, a count known as sefirat ha’omer. There is a machloket throughout the poskim whether a katan that became a gadol during this period or one who converts can count sefirah. The Shulchan Aruch (489:25) explains that in such a case, they may continue to count without a beracha. The Malbim (Parshat Emor), Chiddushei Ha’Rim, and Avnei Neizer explain that such a katan cannot count with a beracha, since counting with a beracha requires complete days, or temimot, and the days that he counted as a katan do not combine with the days he counted when he became a gadol. Rav Yosef Engel learns that even if a katan was lacking one day of  counting, he can start again by counting with a beracha when he becomes a gadol. This is because from this day he becomes a chayav b’poel and the process of the days counting as temimot begins only when he becomes chayav b’poel. The same would apply to a ger. Furthermore, the Maharam Shik explains that a katan can count with a beracha since the sefira of a katan is considered a mitzvah. This is because when one who is not included in a mitzvah performs that mitzvah anyway, it is still considered a mitzvah for him and would be considered part of the temimot he needs.

The Minchat Chinuch (Mitzvah 306) on the other hand rules that such a person can continue to count with a beracha as long as he did not miss a day of counting when he was a katan. His reason is based on the Mordechai in Megillah (siman 798) who explains that an act performed when one is only obligated mid’rabanan can suffice for a d’orayta obligation. In a similar vein we see one can fulfill his obligation to hear kiddush on Shabbat during a time in which he is only obligated d’rabanan, like plag ha’hincha. Therefore, since a katan who became a gadol at least reached the age of chinuch beforehand, his chiyuv drabanan can apply to continue the count as a chiyuv d’orayta. However, the Maharam Shik explains that this point depends on the machloket whether the katan is obligated in the mitzvah of chinuch himself or his chiyuv only comes through his father. This point is a machloket between Rashi and Tosafot (Brachot 48). According to Rashi, the chiyuv is on the father and not on the katan at all. Therefore, it seems that he does not have the same level of obligation as a gadol, and he should not be able to be motzi others through his father’s chiyuv. According to Tosafot, the chiyuv is on the katan himself, which would imply that he is able to be motzi others.

The Gemara in Berachot (20b) says that women and children can be motzi men in birkat hamazon as long as the men ate only a k’zayit and are therefore only chayav mid’rabanan as well. However, when a man eats a full meal and is therefore chayav mid’orayta, a woman or katan cannot be motzi him. Rashi explains that a katan is not chayav at all, even d’rabanan, since the chiyuv of chinuch falls primarily on the father and not on the son. According to Rashi, we can understand why a katan cannot be motzi a gadol in birkat hamazon and sefirat ha’omer, since the chiyuv is on the father and not on the katan himself. However, Rabbi Akiva Eiger (on 48a) asks why women cannot be motzi men in their chiyuv. The Kuntrasei Shiurim (Nedarim 4:3) answers that a woman’s chiyuv is considered a completely separate chiyuv than that of men. For a katan, he is at least included in the mitzvah of men through chinuch.

The Kehilat Yaakov similarly explains that in order for one to be motzi someone else, he must be considered a bar chiyuv and chayav b’poel. A katan is a bar chiyuv and is really considered b’oto davar which means he is included in the kiyum d’orayta, but he is not chayav b’poel. Therefore, he can not be motzi men. However, women are not part of the chiyuv birkat hamazon since there is no kiyum d’orayta, and therefore they can not be motzi men in their chiyuv.

According to Tosafot, we understand why women and katanim have the same status since their chiyuv d’rabanan is on themselves and not a chiyuv from anyone else. However, it is more difficult according to Tosafot, why a katan can not be motzi a gadol in his chiyuv, since a katan is considered both a bar chiyuv and chayav b’poel. We can give an answer by saying that even though a katan is chayav b’poel and a bar chiyuv, since he is only a chayav b’drabanan, he can not be motzi a chiyuv d’orayta.