Andrew Cunningham for Ars Technica:
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. The best you can do is look at historical data. 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. 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.
Although Snow Leopard (OS X 10.6) was not affected by the SSL/TLS bug that affected OS X 10.9 Mavericks, other versions of OS X received security updates as well. Besides the fact that all but the oldest machines running 10.6 can obtain and run 10.9 for free, it’s pretty obvious where Apple would like to see its customers move. Although I still think 10.6 was one of the best releases of OS X, lack of support for a number of things means that it’s probably time to move on. If you’re running an older machine, Cunningham provides some great suggestions on some options.