Brake Pads - when to replace?

[MadMark911]

So my 991 Carrera S is 4.5 years old and only done 12,500 miles. The brake feel is good, there are no warning lights on and the discs are in good condition with little wear, but the dust off the pads is significant even under relatively moderate use.

Yesterday I gave them some proper punishment on my private racetrack - including a very hard stop after the main straight. When they grumbled noisily and promptly filled all of the drilled holes with brake dust. Which gave me an odd feeling that they had gone "soft" (and were breaking up) ..... I

So my question is whether brake pads degrade with time as well as heat cycles and frictional use? I've never had pads this old on a car, so I'm wondering if the pads start to break up when they get older? Anyone else experienced this?