Is Adobe InDesign Server able to open InDesign documents older than its own version?

Yes, but only by a database statement (to increase the app_version of the InDesign documents). After the database statement the InDesign documents will be opened in the newly defined app_version without further request by the InDesign Server. Please contact us to clarify further details to avoid critical converting problems and data loss!

Alternative: Open each InDesign document manually with a InDesign version corresponding to the InDesign Server, confirm the converting to new version dialog and save it again. This increases the app_version of the layout asset, too.
Afterwards InDesign Server will handle those files without further requests.
How to search for specific app_versions in censhare see "Related Content" section below.