Earlier quoted context omitted.
I'm doing an internship at a mainframe shop right now. I took a look into it and the oldest macro I could find that I use semi regularly is SYS1.MACLIB(DCBD). A utility for working with data control blocks which dates back to 1977, putting it at 36 years old. There are definitely older programs on the system, but I'm not sure how often they're used.
I bet IEFBR14 beats it. I also think it wins in the "smallest number of patches per year for actively used software" category.
Re: Ask HN: What's the oldest piece of software still in use?
#81And (apocryphally) "largest number of patches per byte" (http://en.wikipedia.org/wiki/IEFBR14#History_from_the_RISKS_...)