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?