Get database creation timestamp from PostgreSQL?
There's no built-in method to do so, but there are workarounds.

Note however that even PG_VERSION changes when the server is updated to a new major version.

So the only reliable way to store a creation date is to make a one-row table with the date in it.

If this feature is important to your application you could consider funding its development. I think it'd be pretty trivial to implement.