It varies for different regions
E for Everyone - 3 years +, 6 years + Children's games, Racing Games,
T for Teens - 12 years +, 13 years + Godzilla: Melee Games, SW: Battlefront games
M - 16 years +, 18 years + Halo, Gears of War
AO is just AO i think, Not sure about any games with AO rating.
Something like that I think.