How do I find the creation date of a user in MongoDB?
As at MongoDB 3.2, you are correct that the admin.system.users collection does not maintain metadata such as created or modified timestamps.

You can add metadata manually using customData, but as you noted this requires coordination with any applications or tools that are allowed to create users in your MongoDB deployment:

use admin
db.createUser({
     user: "bobbytables",
     pwd: "CorrectHorseBatteryStaple",
     customData: { created: new Date() },
     roles: [
        { role: "readWrite", db: "accounts" }
     ]
  })


Ideally this sort of metadata would be maintained by the database server in a standard format that tools could also choose to interpret. There is a relevant open feature request that you can watch/upvote: SERVER-10021: Add timestamps to user documents.

If you are using MongoDB Enterprise there are a few extra features that may help you with a workaround in the interim:


You can use an external authentication mechanism that maintains metadata for users, such as Kerberos or LDAP.
You could potentially enable auditing to capture createUser and updateUser events, and write an admin script to update the customData accordingly.