A simple answer is are they old when you measure them, or when you listen to them? Odds are you can measure changes before the ones that you can hear. I have a 32 year old APT 1 amplifier that sound good but the caps would invaribly test poorly by now, though.
And to that end, heat and years do add up. A hot class A amp will more than likely fall offs it performance curve before a cool running class D amplifier.
I agree with the 20 year "servicable" life. Anything after that is gravy with electrolytic caps. But MANY a component goes way past that. Capacitors are like valve tubes, same type tubes can go in a year, some decades. Same tube, different life. On solid state change years to decades and decades to 20 years.
And to that end, heat and years do add up. A hot class A amp will more than likely fall offs it performance curve before a cool running class D amplifier.
I agree with the 20 year "servicable" life. Anything after that is gravy with electrolytic caps. But MANY a component goes way past that. Capacitors are like valve tubes, same type tubes can go in a year, some decades. Same tube, different life. On solid state change years to decades and decades to 20 years.