Australian classification review board has rated GTA 5 and looks like they have slapped it with an R18+ rating.
They found most of the things like themes, violence and language to be a strong impact. But one thing went on high impact as you can see from the chart below.
Recently Saints Row 4 was denied classification since the CRB found it to have a lot of high impact things.
The game comes out on September 17th for the PS3 and Xbox 360.