I've come across code on IBM System i (AS/400) with copyrights from the late 70's. The code still works, plenty of businesses run on it, and it was written quite nicely.
The core code base dates back to the System/38, which was released in 1979. You might expect to find a whole series of older to newer dates in copyright notices for the AS/400 (which is not its correct name now, and hasn't been for ages) code base, but the copyright laws have changed a few times over recent decades and I don't know what the current requirements are.