I don't think there is a way to do this authoritatively. There is no timestamp information stored in the database, and keystone doesn't log any details about tenant creation/deletion.

You might be able to get an approximate value for this by looking at the the Nova instances table; for each instance, there is a created_at and project_id value, and these database entries are typically not deleted even when you delete the instance. So, for example, on my system I can run:

MariaDB [nova]> select created_at, project_id from instances limit 5;

And get:

+---------------------+----------------------------------+
| created_at          | project_id                       |
+---------------------+----------------------------------+
| 2014-07-17 20:49:45 | 4dfe8e38f68449b6a0c9cd73037726f7 |
| 2014-07-18 22:02:10 | 4dfe8e38f68449b6a0c9cd73037726f7 |
| 2014-12-29 21:09:29 | 95d9bbd9b446438a89a353d8adb60704 |
| 2015-01-10 03:22:30 | 95d9bbd9b446438a89a353d8adb60704 |
| 2015-01-21 04:34:45 | 95d9bbd9b446438a89a353d8adb60704 |
+---------------------+----------------------------------+

If you are keeping regular backups of your database, you can of course investigate your backups to determine when the tenant first appeared. This is probably the most accurate method of finding when the tenant was created, but it requires both that you have a backup procedure in place and that your retention policy is long enough to find the required data.