How do you define old? The oldest initial release or the last update?

bash

Because it holds up the universe on its shoulders.

Even Apple's 40 floors of lawyers couldn't retire it.

JOE [0] for editing directly in terminal (Linux, macOS)

Somebody gave me Slackware Linux [1] floppies in 1994 and schooled young me on worlds beyond MS-DOS and Windows 3.11 For Workgroups; I had accidentally discovered MS-DOS Editor [2] by being frustrated one day and typing edit into the command line (while muttering "HOW DO I EDIT A FILE?", barely understanding what a file was in the first place), so I was shown that JOE was like edit but also way better. The UI and keybindings ended up burned into muscle memory, so I just kept using it, and still do -- and there's basically no point in ever replacing it now or starting to use emacs/vi/etc instead lol

[0] https://en.wikipedia.org/wiki/Joe%27s_Own_Editor

[1] https://en.wikipedia.org/wiki/Slackware

[2] https://en.wikipedia.org/wiki/MS-DOS_Editor

Well it's not quite a 'software tool' but where I work we still sometimes have to handle a deck of column binary punched cards.

Notational Velocity's last update was in 2011. Still works like a charm 15 years and 14 macOS releases later.

Circuit Maker 2000, for designing descrete circuits. I've tried to move to LTSpice a number of times, but it's interface is less intuitive. CM200 has a really clean UI for design and simulation. Doesn't run as well on Windows as it used to, but still good enough

vi. Fifty years old now. It's a defacto TUI text editor on any embedded Linux system I work with and I can count on it to always be there. No replacement comes close, or has a smaller footprint in storage.