One option, although not necessarily the only option, would be to write a script/program to scrape the REST APIs. You would need to list the projects, and for each project list all the repositories. From memory Stash doesn't track the created date of repositories so you may need to keep your own
https://developer.atlassian.com/stash/docs/latest/reference/rest-api.html
You could also create a plugin which would do roughly the same thing, but being able to bypass permissions and run faster (avoiding all those network calls).
https://developer.atlassian.com/stash/docs/latest/how-tos/creating-a-stash-plugin.html
Finally, the above suggestion to look at the repositories on disk, on the Stash server (assuming you have access) may be trivial because you'll have the directory created timestamps as well.
https://confluence.atlassian.com/display/STASH/Stash+home+directory
You can also access that directory from a plugin if that helps avoid accessing the server directly.