@Cylab: Will that actually do any good in this situation? Looking at the javac version 7 options, I found the following:
Generate class files that target a specified version of the VM. Class files will run on the specified target and on later versions, but not on earlier versions of the VM. Valid targets are 1.1, 1.2, 1.3, 1.4, 1.5 (also 5), 1.6 (also 6), and 1.7 (also 7).
The default for -target depends on the value of -source:
If -source is not specified, the value of -target is 1.7
If -source is 1.2, the value of -target is 1.4
If -source is 1.3, the value of -target is 1.4
If -source is 1.5, the value of -target is 1.7
If -source is 1.6, the value of -target is 1.7
For all other values of -source, the value of -target is the value of -source.
From that it would sound like you could limit your source files to use only features available in Java 6, but the minimum target platform is still going to be Java 7.
@nerb: Java is backwards compatible, not forward. Trying to run a newer .jar file in an older JVM usually causes Java to throw an exception on start-up. I can't recall the exact wording off the top of my head, but I've seen it happen plenty of times where I work when we deploy newer tools.
@Sinuath: You can actually have multiple JDK's on the same machine at the same time. It's a bit of a pain to get set up at first, but I can say from experience it does work. The third answer down (and a few below that) on this SO page
explains what needs to be done.
A much easier option would be to require Java 7 to play the game. If you're worried about end users not wanting to install/update their JRE, you can package a standalone version of Java with your application. There are more than a few threads around here explaining the process.