Why are three year old eclipse glasses not recommended? NASA's eclipse safety page says that eclipse glasses should:


  Not be used if they are older than three years, or have scratched or wrinkled lenses


Why shouldn't three year old glasses be used? Is it because the coating somehow degrades with time? Were standards changed three years ago such that the older glasses can no longer be recommended? Is there just a stronger concern for scratches or damage over the years? Will eclipse glasses (or other solar filters) purchased today be unsafe three years from now?
According to the American Astronomical Society (AAS), the warning that eclipse glasses should not be used if they are more than 3 years old is outdated (provided that they are compliant with the ISO 12312-2 standard).

A page on their web site says the following:


  Note: If your eclipse glasses or viewers are compliant with the ISO 12312-2 safety standard, you may look at the uneclipsed or partially eclipsed Sun through them for as long as you wish. Furthermore, if the filters aren't scratched, punctured, or torn, you may reuse them indefinitely. Some glasses/viewers are printed with warnings stating that you shouldn't look through them for more than 3 minutes at a time and that you should discard them if they are more than 3 years old. Such warnings are outdated and do not apply to eclipse viewers compliant with the ISO 12312-2 standard adopted in 2015.