Dude, Halo 3's an awesome game on every single level.
The rating was a 9.6 on average; everything about it- the graphics, the gameplay, the mechanics, the music, the multiplayer, the singleplayer, the story, the enemies, the weapons, the sound- were basically perfect.
When you're playing it, it's seriously different than any other fps out there- you more than care about the characters- its like ur actually fighting to save planet earth, and u feel that when you're playing in single player.
It's rated mature for blood, violence, the occasional gore (as in a grenade) and strong language- aka cussing.
The violence isn't that bad. it "says" gore, but the most gore they have is something getting shot and blood spraying. you never actually see organs or limbs come off or anything like that.
Yeah I would give it to someone around age 13. Me and my friends played the 1st one back when I was that age, inbetween chillin, sports and other stuff; it's not bad at all really. You don't ever feel like you want to be violent- it makes you want to do the right thing.
So if your kid is wanting to play it, I'd get them it.
Just don't buy them games like GTA, manhunt, or the guy game, or stuff like that.