Isn't 20 years considered the bare minimum for solar panels (with a good warranty lasting that long)? They sure as hell aren't useless automatically around that point- the peak output has certainly degraded but not to the point where you couldn't easily continue using them for 5, 10, or even 20+ more years.
Panels degrade at ~0.5 - 0.8% per year (for various reasons), and are considered candidates for replacement when they've deteriorate to 80% of rated capacity. See the NREL study referenced elsewhere in this thread.
I do suspect that extending the life of solar plant may well help reduce total costs.
Omni's point is that they are "considered candidates for replacement" by companies with a vested interest in selling you replacements.
I'm sure that car makers consider a 3 year old car a "candidate for replacement" as well. Doesn't make it worthless or not worthy of continuing to run rather than investing in replacing...
