I am 17 (almost 18, so I selected that I am above 17). Believe me, you have to play it in order to give a good review, and I played the campaign twice, then played for about 30 hours in multi-player mode. Ok, there are some negative aspects to this First Person Shooter. If you play this game in campaign mode, you run into aliens all over the place, so it is constant shooting and action (this is an aspect of the game that I liked, however, everyone has a different viewpoint). This part of the game is something that I would allow a 10-year old to play. However, it does not stop there. Later on in the campaign, you run into an alien-like civilization called the Flood. The Flood is a bacteria-like creature that lives by taking over aliens and Marines, and turning them into strange-looking, tan-colored creatures, they multiply inside the aliens and Marines, then burst out, making more of them. This point in the game is slightly strange and some people would view it as scary. This is what makes it in my view a game for 12-year olds and up. In multi-player mode, it is a game in which you are a master-chief (The main character in Halo, a heavily armored person with the ability to self-heal) who faces other Master-Chiefs in mortal combat, using a variety of weapons, including sniper rifles, plasma rifle, frag and plasma grenades, shotguns, and more. In multi-player mode, the person with the fastest reflexes is the person who wins. If you were to play multi-player only, I think that 9 or older is age-appropriate.
Here are my ratings on the game for each item.
Single player campaign:
Violence-3.5 of 5
Language-3 of 5
Blood-4.5 of 5
Gore-4.8 of 5 (almost non-existent)
Age-12 and up
Violence-4 of 5
Language-5 of 5 (It matters how much your friends swear while playing, but it is non-existent in the game itself)
Blood-4.5 of 5
Gore-5 of 5
Age-9 and up
Mature-17+ ??? come on, this game in no way deserves a mature rating, it easily gets a teen rating in my book, and if you play multi-player only (like most people do), then it can easily get a E 10+ rating.
Highest the rating should be is Teen.