If a bar mitzvah boy is born on Tisha B’Av, and he eats, כדי שביעה, the amount that fills him up (which obligates an adult in birchas hamazon on a de’O’raisa level) during seudas shlishis on Shabbos erev Tisha B’Av and finishes just before shkia, should he recite birchas hamazon immediately and fulfil his chiyuv on a rabbinic level, or should he be worried for the opinions that maintain that the chiyuv birchas hamazon goes after when the food is digested (זמן העיכול) which will be after tzeis hakochavim when he is already bar mitzvah, and he will be obligated in birchas hamazon on a de’O’raisa level after tzeis hakochavim?
R' Akiva Eiger (Orach Chaim 186) and the Shu”t Kesav Sofer (Orach Chaim 31, and 99) are in doubt if we go after the time of eating which was when he was a kotan, or if we go after the time of digestion which is at the time he is a gadol. The Chazon Ish (Orach Chaim 28, s.k. 5) leans more towards the possibility that the chiyuv stays rabbinic, his argument is: Since there was a time in-between the eating and the digestion when it wasn’t de’O’raisa, it can no longer become de’O’raisa, however, he concludes with tzorich iyun.
The Shu”t Shevet HaLevi (8:56, s.k. 2) writes: Logically it seems more like the opinion that he is obligated to recite birchas hamazon on a de’O’raisa level, as even though at the time he finished eating he was still a kotan, since he benefits from being satisfied and from his eating when he is a gadol, it makes sense to go after that time. However, he writes that logic doesn’t help determine the shailah and there is still a doubt, however, certainly he has a rabbinical obligation, and even though when he was a kotan he also had a rabbinical obligation, that rabbinical obligation was only because of chinuch, however, now that he is a gadol the rabbinical obligation is more sincere.
Regarding the doubt of R’ Akiva Eiger and the Kesav Sofer, the Shu”t L’Horas Nossan (7:14, s.k. 10) points out, that it’s difficult to entertain a possibility that the bar mitzvah boy must recite birchas hamazon a second time after tzeis hakochavim when he becomes a gadol. If we check him for two hairs and we find he is a gadol, then perhaps he would need to repeat birchas hamazon, however, if we don’t check him, how can we obligate him to recite birchas hamazon again because of דרבא חזקה, normally דרבא חזקה doesn’t work for de’O’raisa’s. Consequently, we should be worried that he is still a kotan and repeating birchas hamazon would be levatolah. In light of the above, it would seem that what the bar mitzvah boy should do is recite birchas hamazon half an hour after shkia, to avoid any doubts (Sefer Chinuch HaBonim LeMitzvos V’Dinei Kotan, R’ Noivert, he’orah 251).
