If I were the judge, I would grant summary judgment to the teen without hearing the case. "So Illinois and Madigan believe the second amendment to be inapplicable to persons under 21?" "Judgment for plaintiff! Next!"
So a 16 year old can operate a motor vehicle, and motor vehicles slaughter people by they hundreds. ...
If she was a he she definetly would have a case, because per federal law she would be eligible for the militia, IMHO.