I think that the non-melting sugar I used on my Pfeffernusse was 2019, and it was fine. My sparkling sugar is probably as old as yours, but I bought it in a bag, then moved it to two empty sparkling sugar containers I had saved, so I have no idea how old it is, only that it works! We shall see if the pearl sugar, which is at least six years old works when I try it on a recipe this weekend.
The rum balls sound delicious.