In a blog post Microsoft announced that they have started accepting games with a rating of PEGI 18.
The first two games to be offered will be The Witcher, by CD Projekt, and Grand Theft Auto IV, by Take Two—with more games coming soon. Unfortunately both games are Desktop, not Windows 8-style titles.
Minors will remain protected by the integration with the Windows 8 store of Microsoft Family Safety, which help parents control the kinds of apps their children can install.
Microsoft emphasized that due to this the Store continues to be a safe and positive place for children to explore.
Read more at Microsoft here.