The oldest *big* working sat is probably TDRS 3, launched 1988, although it is basically in storage, same for a couple of late 1980s NRO sats. The oldest large (> 1 tonne) Earth orbiting satellite that is still in operational service rather than reserve storage is HST!

That’s awesome. I’m doing research on behalf of an organization that maintains lists of superlatives, and I’m trying to figure out if there’s any satellite doing science (or whatever work it is supposed to do) that has been operating longer than Voyager 2

Oh god. I'm old.

My dad worked on that series, starting with TDRS-A which was renamed to - 1.