Which one should be running anyway, seeing that 10.9 is the current version for a year, and we're on to 10.10 soon.

OS X releases are like minor Linux distro updates -- not in they have limited changes, but in that you don't want to fall far behind the latest version.

It's a different approach than Windows, where people can still run XP with no problem -- not that you couldn't run 10.4 or 10.3 (and some people do), but the Mac world changes and software is updated to reflect that (takes advantage of new APIs, integration, changes in style etc).

In Windows there is software available in new versions in 2014, even MS made, that still uses stuff like file open dialogs from 1998.