When OS X 10.9.2 was released earlier this week, it was accompanied by security updates for OS X 10.8 and 10.7 but not for 2009's OS X 10.6. Since switching to a yearly release cadence with Lion back in 2011, Apple seems to be willing to support whatever the latest version is, plus the two preceding versions. The best you can do is look at historical data. Apple offers no end-of-life roadmaps for its operating systems, and it doesn't officially comment on whether support has dried up for this or that version of OS X.
