This is a response to criticism that the Android Market is perhaps a little too open, although one of its attractions is the fact that it isn't as closed off as Apple's App Store. Back in April, Steve Jobs recommended Android as the place to go if you wanted porn.
Android has a content rating policy already, but it isn't visible to users, and apparently not strictly enforced. Now adults and kids alike can distinguish much more easily whether the app they're looking at is suitable. If developers decide not to stick a rating on an updated or new app, it will be automatically marked in the group Mature.
There's obviously a danger that developers may give their adult content a lower age rating than it should have. And people may have different opinions on how an app should be rated. For example, the boundary between Pre-Teen fantasy violence and Teen realistic violence can be blurred.
Android Market staff may re-rate the apps if users complain they're incorrectly rated, according to Mashable. You can't get away with apps containing hate speech or porn, although suggestive or sexual references are permitted.
Apple has been classifying games since 2009, with 4+, 9+, 12+ and 17+ ratings. Ever since iOS 3.0 you've been able to bar an iPhone from downloading apps that are above a certain rating.
The main difference between the App Store and the Android Market as far as age ratings is concerned now seems to be about company input. Google will wait for user feedback before reactively changing a rating, whereas Apple actively vets apps before they're published.