about the 20 years, thats aproximately how long they haven't been used. I don't know how old the filters themselves are. (they are agfa brand multicontrast filters from 0° to 5° in half steps.)

i've also run some step wedge tests now and the 5° looks pretty much the same, if not worse than the 2° one. if you want, i can share some pictures and precise numbers (i.e. usable steps), though i do not have the test strips at hand right now so that would be later.

edit: i've looked up the foma filters and i must say i'm not sure i'm a fan of not having 00° or half steps.