Do you say Boruch Shepatrani for a girl at her Bas Mitzva? If not, why not?
The Yalkut Yosef says that it depends on the reason for saying the blessing:
We say it because the boy will no longer be punished for the father's sins (and [underage] girls are punished the same way [underage] boys are).
We say it because the father will no longer be punished for the boys sins (as he is no longer responsible to educate him). (And one is not obligated to educate girls to the same way as boys).
I don't remember his final conclusion, but (at least in the Chabad world), I have never heard of it being said by a bas-mitzva.