It's mostly a function of the effort put into the quality of the code base. Currently, I work (among other things) on a 25 year old code base that acts as an i

It's mostly a function of the effort put into the quality of the code base. Currently, I work (among other things) on a 25 year old code base that acts as an interactive, terminal-based interface for a CMDB. It has its problems (like mostly not using the exception mechanism of the programming language it's implemented in; probably wasn't very reliable back then, or maybe didn't exist), but all in all it's OK to work with. Most changes touch only 1 to 3 files, most of which are pretty short (<100 lines of code, typically). There are "here be dragon" areas, but not too many.