1. Go to the Admin cog and under the Property column, click on Tracking Info and select Data Collection.
2. Next, you'll opt in to Google Analytics tracking cookie for advertising under Advertising Reporting Features. Click on and Save.
3. After that what you'll do is go back and click on Property Settings in the middle column.
4. Scroll down until you see Enable Demographics and Interest Reports. Go ahead and turn that on. Go to the bottom of the page and click Save.
5. You can now go under Audience > Demographics > Overview and you'll be able to see age groups, gender, and interests.
Note: If you just enabled this feature it might take a day or two for the data to appear in these reports!
You have now enabled demographics tracking in Google Analytics!