Mind if I add my two cents worth to this topic?
Australia's rating system consists of the following:
G - Suitable for all ages
PG - Parental Guidence Recommended
M - Recommended for Mature Audiences
The following are legally restricted ratings:
MA +15 - People under 15 must have a parent or adult guardian
R +18 - Restricted for adults 18 and over
X +18 - This only applies to the states ACT and Northern Territory, and applies to pornographic content.
Also, video games that exceed the MA +15 rating are refused classification (RC), unless they are edited and resubmitted for the MA +15 rating. There's no R +18 or X +18 rating for video games.