how do I see when a cloudsql instance was created
Solution 1:
If you click on 'Operations' in the left CloudSQL menu, it should have a list of all operations your database instance is currently running and has completed, the very first on should be of type "Create" and should have a timestamp.
UPD:
you can also try to get this info with gcloud command: gcloud sql operations list --instance <instance_name> --filter='operationType=CREATE'