Firebase Analytics demographics

On Android, this data is derived from the Advertising Id, which is automatically collected on devices with Google Play Services installed. On iOS, this data is derived from the Advertising Identifier (IDFA) when available (i.e. when your app links to the Ad Support Framework). AdMob is not a prerequisite for Age, Gender or Interests on either platform.

Clarifying note from comments: Firebase Analytics demographic reports is thresholded for privacy reasons. Once you have enough data -- at least 10 users per age/gender bracket -- data for that bracket will be shown.

In case someone else stumbles upon this, make sure that you have the google signals enabled in the admin console. This is not part of the firebase analytics onboarding process as far as I remember, and it's off by default.

You can find how to do it here:

https://support.google.com/analytics/answer/9445345?hl=en