halo 3 is M for mature i thought that mean 17+ automaticly.
AO is only game rating for 18+
We follow the European game rating system, a.k.a. PEGI. This applies to Europe and the rest of the world. The American system is different to PEGI, if I'm not mistaken, the ratings are Everyone, Teen (13+) and Mature (17+).
For PEGI, there is 3+, 7+, 12+, 16+, and 18+. ES would be familiar with them, too. Halo is a 16+ game for us PEGI-followers.