Determining which Mercurial version which created a repository

I have a rather large Mercurial web server (ie. running under hgweb.wsgi) that's outgrown the distribution it was built-on... which basically means that I've already started doing upgrades of Python, Apache, OpenSSL, mod-wsgi, and Mercurial (among other things), rather than being more-responsible and upgrading the underlying O/S (don't hate on me).

The question here, is... what is the best way to determine which version of hg actually created any given repository. Ideally, I'm looking to get rid of the older database versions and help make sure those repos basically "keep up" with current versions of Mercurial.

I understand the general .hg/requires file, which can give you "compatibility requirements," but that does completely answer the question of "how old was the version of Mercurial that created this repo?"

That said, there's probably some place that lists the "new" requirements for any particular version, so you can artificially construct a version matrix? Unfortunately, I was unlucky on the mercurial-scm.org site in finding anything definitive.