Oldest computers still running production code?

Voyager 1 & 2: https://en.wikipedia.org/wiki/Voyager_1

Satellites in general. Once you've built out a ground infrastructure for a spacecraft, it makes zero sense to ever make dramatic changes to it. The risk is too high.

In 2015 I worked on a piece of software that was related to command and control of the Cassini spacecraft that ran on Sun Solaris machines. That infrastructure would have been first brought up prior to launch in 1997.

Granted, I have to assume that these were not the original physical computers and that there are still ways even today to source new solaris machines for exactly this type of scenario, but still.

Bombs. There's no shortage of actively deployed weapons that rely on unmodified 60s and 70s technology. Iran still operates F-14A Tomcat jets that presumably rely on their original 1970s digital avionics. Hell only knows what kind of mess Russian engineers deal with operationalizing Soviet-era digital weaponry. America should rely on more modern weapons integration (with the focus on NATO-standardized Datalink and digital fire control), but I'm certain there are still stockpiles of usable 70s and 80s era digital weapons laying around.

All of them are expected to be deployed to production and function flawlessly.

Systems that run factories. In 2010 I worked on a project involving a PC running some DOS-like OS from 1983.

The PC software controlled the massive ovens which are used in the formation of soft drink plastic bottles.

The gear shop I worked in hade a 50 Horsepower CNC lathe that was controlled by a 1970 GE computer that stored data on punched mylar tape. It ran until 2019 when the shop was acquired, and it wasn't part of the move.