My instructions say the JDK version "must not have been superseded by a new production version for more than 18 months by the time you make your submission." JDK 5 was superseded by JDK 6, so you have 18 months from the release of JDK 6 before JDK 5 becomes too old for use here. Unfortunately I don't remember exactly when JDK 6 came out, but I'm sure it's less than 18 months ago. Be sure to use a recent version of JDK 5 though, since (for example) JDK 5.0 Update 1 was superseded by Update 2 a long time ago. It's unclear whether Sun cares about minor updates "supereseding" each other (I think maybe they just mean major versions), but it's best to be safe.
Having said that, upgrading from JDK 5 to JDK 6 is pretty simple, isn't it? I don't see any good reason not to simply use the latest JDK version here.
I wrote Sun with this very question, because I found the 18 months thing very confusing.
Below is the main answer I got:
"While certification exams, such as the SCJP 1.4 and SCJP 5.0, are designed to test for specific versions of Java, the SCJD assignment is designed to be completed using a current version of Java. You may actually use either Java SE 5.0 or 6.0 for the SCJD assignment.. Java SE 6.0 was released December 11, 2006, so Java SE 5.0 may be used for the SCJD assignment until June 11, 2008."