[bruce511]

Panels are often quoted has having a 20 year (or 25 year) lifespan. But it doesn't really work like that.

Panels degrade a bit (about 0.7%) every year. So after 25 years or so they're down to 80% of rated power. Or, put another way, after 25 years they're still delivering 80% of rated power.

Depending on space, it may be advantageous to replace panels at some point. Or you might add more and leave those alone.

But they don't just "stop working" - the performance drop off is pretty linear.