Violent, but not bad.
AOE III is certaintly not for everybody. Parents should know that it is a game all about building up a civilization to crush another one. The only way to win is genocide. However, the violence isn't the kind of thing that I believe would alter someone's view on war in a violent way.
You should make sure that your child can handle the large explosions and mass deaths. However, if your child wants this game, chances are they are LOOKING for a challenging but fun RTS game. They'll be ready.
I believe that any Tween is mature for this game and quite possibly those in the single digits.