title: How to use default user properties with firebase Analytics
body: <p>I've read on the doc that firebase uses the Android Advertising ID or Apple IDFA to get demographics (age, gender) however this information doesn't show up in the analytics.</p>
  
  <p>Is there some additional steps that need to be done in order to activate or link these informations to the Analytics? I'm currently using some custom userProperties but it would be good to know why the default ones doesn't work.</p>