You can compile for older versions of the JRE by setting the compiler target compliance level. This allows you to generate class files that are usable on older versions of the JRE as well as the current one.
From your IDE, you will be able to select the compliance level, but if you're compiling manually using javac, this can be done by setting the appropiate command line parameters: -source and -target.
For example, to compile for JRE 1.6, do this:
javac -source 1.6 -target 1.6 MyClass.java
A quick explanation:
You should always try to specify both source and target compliance levels. They are usually set the same.