It’s good to know that they have pretty good longevity. One thing complicating this is that panel technology has gotten better and better during that time. There’s a graph on Wikipedia plotting how much better the various types of panel have gotten since the 70s. A lot of them have doubled in output since the early 90s.
So on the one hand, these old panels are outputting 75% of what they started with, which is good. But on then other hand they are only outputting about 37% of what new panels could.
Not that we should throw old panels away. There’s plenty of sun to go around (though I guess the average homeowner only has one roof to use). It’s just interesting how fast the tech has improved and how that might factor in to some longevity calculations.