I know I’m beating my head against a wall with this… but I was trying to connect windows 98 up to jenkins as a slave just as a dumb challenge.
Got jdk 1.5 install but hit unsupported class version error, when trying to connect via jnlp. So what’s the oldest version of the agent.jar I can get working with jenkins that works with either jdk 1.5 or jdk 1.6?
We can barely support the most recent of releases so I wouldn’t expect much.
I believe we’ve been java 8 maybe 7 since 2.x so probably early 1.x line, or maybe even Hudson
Hello @MikeyCarter and welcome back to the community 
You may want to try installing a later version of Java: Sun Java Runtime Environment - KernelEx Wiki. .
Good luck with that.
Java 1.5 is the last supported release on Windows 98 (ie: 1998, or 199-05-05 for 98SE)
Not supported on SE 6.
The last public release of Java 1.5 was jdk-15_0_22, 2009-10-09
And the Extended Support release was jdk-15_0_85, 2015-04-09,
Though probably not on Win98 SE since support for the OS ended on 2006-07–11.
The oldest Jenkins LTS release is 1.409.1 (2011/06/06) , which is two years after the the last public jdk1.5 release
This commit from Koshuke, Jun 15, 2013, pushed Jenkins core to jdk1.6. It was tagged 1.565.1-rc1, released 2014-07-30
Less than a year later, Koshuke and Jenkins said goodbye to Java 6.
So, without hacking things, I’d say you are not likely to get anything newer than 1.565.1 (so 1.554.3 (2014/06/30) ) to work at all.
And my dumb challenge was pulling this response together!