Still older Macs can’t even be upgraded to Lion, meaning they’ll be stuck with Snow Leopard (version 10.6.8) or some earlier version of OS X.
In recent history, Apple has only patched operating system vulnerabilities for the current and two previous versions of OS X. However, some Macs are still limited to Lion (version 10.7.5), which is evidently no longer getting security patches now that Yosemite has been released the lack of a Lion version of the recent Security Update 2014-005 is a harbinger of things (not) to come. This means that if your Mac was compatible with Mavericks or even its predecessor Mountain Lion, you’ll be able to upgrade to Yosemite. Like last year with the release of Mavericks, Apple chose to continue supporting all the same Macs as the previous release of the operating system. Apple recently released a new version of its Mac operating system, OS X Yosemite (version 10.10).
