Linux license
 

I have a license for RHEL 6.0 but my application runs only on RHEL 5.5. Will I be able to transfer this license to RHEL 5.5?

If you have a license, ask Red Hat.

AFAIK, a license / account is valid for all RHEL versions.

RHEL license
 

The license is for RHN and RHEL support. It is not tied to a version. They publish the end-of-life information (that date after which they will no longer develop, update, or provide patches, for a version) well in advance. Your 5.5 should be in support for many more months.

You can also explore why it is that "the application only runs on Red Hat 5.5." What is the technical reason for this?

One very significant feature of Linux is that there can be many distinct versions of any given library installed on the computer at any time, and any application can be either as specific as it wants to be, or as generic as it wants to be, in stipulating its requirements. (If you want "version 1.2.3" you can do so. If you want "version 1.2 or later," you can have that too.

Probably, if your app needs, say, an older-version library or somesuch, it can be installed.