There are at least two sections in the Windows 8 app certification requirements, that are related to the content and the age rating of your app, and which provided good bases to reject some applications:
5.1 Your app must not contain adult content, and metadata must be appropriate for everyone
Apps with a rating over PEGI 16, ESRB MATURE, or that contain content that would warrant such a rating, are not allowed. […]
6.2 Your app must have a Windows age rating […]
[… ] 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+. […]
In other words:
- You must not have any adult content in your app (Windows Store does not support that).
- If you app connects to any data source that provides content from other users, you should select 12+ age rating. So if your app is a Twitter client (like WallOfSilver), then it must have 12+ age rating because anyone can write anything to Twitter, and it can be displayed by your app. According to community experiences, it’s better to select 16+ rating.
If you need, you can even try to filter the content, using for example the List of Dirty, Naughty, Obscene or Otherwise Bad Words dictionary. Do you know any other source or service for that?