It's 26 years for me. Emacs is I believe the oldest software I still use. I started on an SGI Irix in 2000. I used it also on HP-UX, Solaris, Windows, MacOS, and of course all varieties of Linux
> Emacs cursor movement keystrokes are quite widely supported elsewhere too which use GNU readline or implement at least subset themselves.
And many keystrokes work on MacOS, too. That was a pleasant surprise when I got a Mac laptop for work.
Agredd! Aside from the basic Unix commands (ls, cat, etc), the only substantial program that I use today that I also used in 1991, is emacs.
Well, and perl. I don't write much perl these days, but I have tons of perl scripts accumulated over the decades that I use daily so that counts as using it.
Third oldest for me would be mutt, which is still my primary email client. But I only switched from elm to mutt in.. not sure maybe 1996?
That is a separate discussion from whether you should use ed or vi normally. You learn the "fallback" with the hope of never being forced to used them.