The Mishnah’s statement that children are not involved in zimun prompts the question of one who eats a meal as a child but has reached majority by the time birkat hamazon commences (see Resp. L’Horot Natan, VIII, 9). Similarly, R. Akiva Eiger (Chiddushim to O.C., 186) questions what rule to apply to a minor who eats and recited birkat hamazon, then reaches majority while still satiated; should this individual, now biblically obligated in mitzvot, repeat the birkat hamazon (a similar question is posed about a bar mitzvah boy who brings in the Shabbat of his birthday early, and recites Kiddush before he reaches bar mitzvah; see Imrei Binah, Shabbat 11; Resp. Binyan Shlomo, drashah l’bar mitzvah; Resp. L’Horot Natan VII, 13-14). R. Akiva Eiger leaves the issue unresolved. R. Chaim Pinchas Scheinberg (Mishmeret Chayim, I, Hil. Berakhot, 6 and 16) notes this is not necessarily comparable to one who eats a meal as an onen, and then leaves that status before birkat hamazon; in that situation the individual is generally obligated, but has a temporary exemption. In the case of a child, the situation is complicated by the fact that many rishonim believe that children are not obligated in mitzvot even on a rabbinical level; rather, the whole obligation is on the father. In this instance, the Chazon Ish (O.C. 28:5; see Hod Tzvi, 1) exempts the newly minted adult, since the active eating was done during a time of exemption. (See Resp. Teshuvot V’Hanhagot, I, 184; Resp. Ktav Sofer, O.C. 31; Resp. Tzitz Eliezer, II, 2; Resp. VaYa’an David, 184; and Yeshuot David, O.C. 10).