If you are under the age 13, certain features are hidden such as the store, extra maps and online mode.
You set your age when first opening the app. If the age you put in was wrong, simply log out through the settings menu, be sure to have already made a password for your account if you want to keep progress. You will then have the chance to put in your age again. Put in your correct age and log back in.
The store should now be visible to you.