Launching today alongside Windows 8, the Windows Store includes various guidelines that prevent ‘M’ and PEGI 18 rated games appearing on store. That includes Microsoft’s own products, such as Halo 4.
As it stands, the issue is not the nature of the content itself, but the way it is classified in different areas of the world. In Europe, for example, the PEGI 18 stamp covers games that the US either rates as M (for mature) or AO (adult only). The latter is essentially a death penalty for many games, with some stores refusing to stock such titles.
Until the red tape is removed and common ground is found, ‘mature’ games will not be available. The content time frame for M-rated content becoming available on the store end of 2012.