Does performance of PV inverters degrade over time?
I don't think the inverter would appreciably degrade, since they tend to be more ruggedly designed than conventional power supplies (no electrolytic capacitors, for example).

As for the panels, other sources claim that panel manufacturers cite a 20% drop in panel efficiency over a 25 year period. This appears to be due to breakdown of the panel material itself.

If you live in a dusty environment, surface dirt on the panels can degrade their performance significantly, but this is reversible with cleaning.