“The boy undergoes a change in his public mitvzah obervance (Tefillin, minyan, etc) and this warrants a public celebration. The girl’s change is in private observance, and this does not warrant a party.”
That was extremely interesting and educational. I would propose that a girl also undergoes a public change (though clearly not as obvious as a boy), because she fasts publicly when required by halacha, and though her observance is otherwise mostly indeed private, she is equally responsible to fulfill the mitzvos as the boy is. And though something perhaps may not necessarily warrant a party, it is nonetheless permissible and a nice thing to do for a girl, to mark the occasion. I still believe one should not go overboard, however.