Early departure

The airline specified a boarding time of 5:00 and the plane leaving at 5:30, at 5:20 the plane had already left and the flysafair denies responsibility completely. They say that the boarding gates close 15 minutes before departure, but i have been on 3 of their morning flights prior to this and not one of them had closed the boarding gates early. There was no warning from the airlines side and the response from the ground hostesses was that theres nothing they can do, we should watch our plane leave.