Geoff Streeter, one of the developers at dyalog.com says:

"I suspect that the oldest untouched code (of mine) in the interpreter is the code for balancing a symbol table. This is going to be early 1982 and is untouched since it was written. It implements a technique from Colin Day and published in the Computer Journal in 1976."

Dyalog's APL interpreter is still profitable and actively developed. Many of the source files begin with:

    /*	Copyright (c) 1982 Dyadic Systems Limited	*/