It could be any number of things. The first thing I would check are all the yellow wire connectors under the seats. Sometimes this wires get damaged or disconnected after years of use (seat sliding back and forth, stuffing things under the seats etc). Check those first. If all is good and the light is still on, the you will need to go to a professional who deals with air bag issues.
In my case (2011 model) it was the "clock spring" cable in the steering wheel - this is a common cause of airbag fault lights in vehicles in general, as it's a mechanical component that eventually wears out.
It's not expensive but certainly not cheap (mine cost me the equivalent of about $180 incl. labour), so yeah probably a good idea to take it to a reputable auto-electrician for diagnosis. As I'm not an explosives expert, I left this to someone who knew what they were doing.