Iggros Moshe OHC 4:36.
A Bas Mitzva is not a Seudas Mitzva, and therefore should not be celebrated is Shul, but it is a Simcha, and a kiddush SHOULD be made, and if the girl wishes to speak, she may do so, but not from the pulpit.
Maxwell took the last paragraph of the Teshuva while ignoring the first two paragraphs. The last paragraph is talking about making a seudah like one might do for a boy (which Rav Moshe is also against).
Did you see it inside or did you just quote someone else?