Our oldest running systems are from 1994 (previous to that date we only used linux for special projects).
Curious as to what systems you're running... your HN bio sounds like an interesting backstory
In this case, telecomms hardware (there may be older systems out there that are off support). Uptimes are often into years; we used to have systems that would run in 8Mb of RAM though the oldest I know of now has 32Mb :)
Uptime of years? How does that work, security wise?
Security patches don't necessarily affect uptime. For example, patching and replacing e.g. libc on a live system is entirely possible, even though it's not the standard procedure.
Its quite possible that for systems that old, there are no updates even available. Its also possible the systems have no route to the internet, which while not making them immune from compromise, certainly makes it more difficult.