The Windows 8 Store terms of service states, "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."
While AO games are never sold in the US in retailers or most stores, PEGI 18 games include major popular titles like The Elder Scrolls V: Skyrim, Mass Effect 3 and others.
It'll be interesting to see if this comes to pass; it's highly unlikely Microsoft would want to bar, say, Call of Duty: Black Ops II from being sold on the store.
Microsoft banning PEGI 18 apps from Windows 8 Store
11 October 2012 | By JonahFalcon