check-out of an older revision should not be allowed

Your system admin has set the system to allow that. By default it is not allowed.

In Windchill Preferences, look under the Create and Edit section.

There is an entry for Allow checkout of non-latest iterations.

There are 3 options:

1) Do not allow the checkout of non-latest iterations.

2) Allow the checkout of non-latest iterations for valid object types like CAD documents and their descendants.

3) Present an overrideable conflict when attempting to checkout a non-latest iteration.

Default setting OOTB is #1.

This setting is not seen at the user level, only at the Org or Site level preferences. Unless you are the system admin (or have admin rights) I doubt you can change it.

Let me add that this can also be prevented by ACL rules that 'lock' rev A in the Release state before creating rev B. Our users cannot check out files in the Release state, they must revise them to the next revision level, which puts the state back to Design.