At this point, FreeBSD 13.4 is the oldest officially supported release. But we've probably all kept a server online for a year or two after it was officially unsupported. Some of us without even rebooting it. Often it's for good reasons, and it isn't even necessarily insecure, as long as we backport the patches ourselves. So what's the oldest release that you're still using? Personal machines count, as long as they're in-use and not merely gathering dust on a shelf. Embedded systems, ala Playstation, don't count, unless you actually have both the ability and responsibility to maintain that OS.