Q. My daughter is going to separate chalah and make a brocho on her Bas Mitzva party, as many others do this days. However, because the vacations start soon, we are making the party early and she will be short a month from being 12. Can she still separate the chalah on the bread baked, that the others will eat?

A. Shulchan Aruch (Y.D. 331: 33) rules that a minor that has reached “Oinas Hanedarim” (the age when a neder or pledge becomes meaningful – for a female that is eleven years), his (or hers) terumah (or chalah) separation is valid. Poskim mention that the above is only “bedieved” or after the fact. (Tsh. Harambam 205, Tiferes Yisroel – Terumos 1: 3. See Hagra ibid. that with the Shulchan Aruch and even after the fact it is not chala).
Minchas Chinuch (507) writes that such a minor, although, after the fact the separation takes effect, since he/she cannot yet become a shaliach or an agent for a full fledged gadol, their act is valid only for themselves and not for others. (See Chanoch Lana’ar p. 80 and Hakatan Vehilchoisov Ch. 54) for divergent opinions).
Therefore, Horav Shlomo Miller’s Shlit’a opinion is that she should first separate the chalah with a brocho as part of the intended ceremony, and then a gedola that is already Bas Mitzva, should separate again for the others without a brocho. Although as mentioned, the Bas Mitzva’s separation should be avoided lechatchila, since there is an issue of kavod habrios involved, as her not being and feeling any less than the rest of her friends in their celebration, it is permitted on the onset.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a