Allegedly, on or about 15 January 2016, Andrew R Paterson sent:
How about the creation date of /etc/redhat-release Works for me!
Doesn't work for me, so you can't guarantee it'll work for others, either.
That file comes from the fedora-release RPM package, which has already been shown to get updated post-installation date, so that file is potentially not going to have the install date.
One could look for the earliest sane date in the /etc directory, though you have to be aware of systems that installed with the date set wrong, and packages that were prepared before installation, and may install with those filedates, rather than current system dates (not sure how it's supposed to work, whether all written files should have a local creation date).
If there isn't anything that's actually meant to record an installation date, perhaps concerned people either want to lobby for it, or simply write some installation notes to file on the day of installing.