The Windows 8 Marketplace will not be able to sell any apps or content with an adulting rating, including video games that are rated 18+, according to the Windows Store guidelines.
Under section 6.2 of the Windows 8 app certification requirements, apps with a rating over PEGI 16, ESRB Mature, or a corresponding rating under other ratings systems as described by the table in the guidelines are not allowed.
A Microsoft representative confirmed with Kotaku that the guidelines also apply to video games.
While this doesn't mean that adult games won't be playable on Windows 8, it does mean that games with adult ratings will not be available from the Windows 8 Marketplace, which could potentially mean that players will miss out on special Windows 8 functionality for certain games.
Kotaku noted that while few games in North America are given a rating beyond "Mature" (the PC version of Manhunt 2 and Grand Theft Auto: San Andreas being a few exceptions), this is not the case in Europe where many best-selling games are given the PEGI 18 rating.
Games in Europe that have received this rating include: The Elder Scrolls V: Skyrim, Mass Effect 2, Mass Effect 3, Dishonored, Deus Ex, Fallout: New Vegas, Assassin's Creed: Revelations, The Walking Dead, Sleeping Dogs, LA Noire, Spec Ops: The Line, Max Payne 3, Call of Duty: Modern Warfare 3, and more.
In Australia, where an R18+ classification is due to kick in on January 1st, 2013, it is also yet to be seen how this will affect the availability of certain games on the Windows 8 Marketplace in Australia.
Windows 8 will release on October 26th, 2012.