She was a passenger in the car when the accident happened on the Magheraknock Road after 8 am on Sunday morning.
The teenager was taken to hospital along with a man whose injuries are not life-threatening.
Her injuries were believed to be serious at first.
However, a spokesperson for the Belfast Health Trust said she is now in a critical condition in the Royal Victoria Hospital’s intensive care unit.
The road was closed between its junctions with Langley Road and Laurel Road but police have said it has now reopened.