What it is
If you let it, Google Analytics will display additional data tying site visitors to data it has already collected (or inferred) about them for advertising purposes, such as age, gender, and interests. This is called Demographic Data collection.
Why it matters
Knowing your users is vital. With demographic data we can see not just what the age and gender breakdown of your users is, but also if any category is being under-served.
What to do
If you have an existing Google Analytics account and it is not collecting this data already, it’s just a matter of switching demographic data collection on if you are using a recent version of the Google Analytics tracking code. If not an additional line would need to be added to the code snippet, so it’s probably better just to upgrade.