With requirement 6.2 of the Windows Store Certification Requirements (v4.0), all apps must have an age rating. Be sure to follow the guidelines because they are taking very serious from everything I've seen, and many apps fail on this simple basis.
One of the key bits to pay attention to is the following statement:
If your app provides a user with uncontrolled: (i) access to online social networks, or (ii) sharing of personal information with third parties, including other gamers or online acquaintances, then you must assign it a Windows Store rating of at least 12+. For such activity to be considered "controlled", your app must include parental control features that require parental permission to use such sharing features, and you must identify those and explain their functionality in the Notes to testers.
To make this story short, if your app uses the Internet, carefully evaluate your usage to understand whether you need the 12+ rating or not. Also see Giving Your App an Age Rating.
You also want to make sure that any terms of service you link to from your app indicates ages that are compatible with your chosen age rating with the Store. One partner I worked with a while back failed for this reason: their terms of service said something about a person needing to be 14 years or older to agree, but the app was rated for an age lower than that. This meant that a young user could acquire the app from the Store but wasn't technically allowed to use it due to the terms of service.
It's good to note that earlier versions of the Store certification requirements specifically called out manifest capabilities like geolocation and webcam as requiring a 12+ age rating in and of themselves. This has been refined to allow apps with a 3+ rating, for example, to use the webcam, so long as it does not allow uncontrolled sharing of pictures. So a 3+ app can take pictures from the webcam and let a preschooler doctor themselves up, no problem, but sharing the results via a social network would require parental consent in the app.