GFCI Tripping after 15 years
There are essentially two possible reasons: 


There is a ground fault in the wiring/fixtures/appliances it's protecting 
The GFCI itself is faulty


It really depends on what's on the circuit to determine if there is a fault somewhere. 15 year old wiring is likely to not be a huge issue. There could be water in a junction box. Simple receptacles and switches aren't too likely to have issues (though it wouldn't hurt to open them to take a look at the connections), but anything with active electronics in it (dimmers, USB chargers, etc) could definitely be the cause. Same goes for other fixtures -- lights, ceiling fans, etc. 

You could replace the GFCI and see if it still happens -- if so, there's definitely a fault. If you don't have another GFCI already, this is a somewhat expensive way to test (though if it's fault, you'll have to buy one anyway). 

If you happen to have another one you could swap in, that would be a cheap and easy way to test. Swap two, see if it still trips and if the same breaker trips, or the same circuit trips the new breaker.