[niklas_a00]

Reason (music making software). Released in 2000 and still an actively developed code base. Still some code from 23 years ago running in production :)

[detaro]

music software is a full field of long-running products:

Cubase: 1989

Pro Tools: 1991

Logic Pro: 1993

FL Studio: 1997

Ableton Live: 2001

Max/MXP: 1980s, 90s as commercial product

[ssttoo]

Also notation software:

Musescore 2002

Sibelius 1993

Encore 1984

Finale 1988