theres no way to really put it into words, so just bare with me please. there are 2 age ratings for every game, theres the higher age (the age you get the game for) and the lower age (the age that can watch and even play the game with the higher age) for this game i would say the high age is 12-13+ (i'm not saying thats the age its made for, just the age its okay for).
the lower age in my opinion is 7-8+, the only reason you might want to watch a little bit of the game before letting the younger aged child play the game is for PG language and 'paint-ish' blood.
language: there is minor PG swearing such as b*****d, d**n, freaking, ecs. (basically what you would find on a family-ish, sit-com) (its so little that they cant even put language on the game rating)
violence, blood and gore: in gameplay your player uses a variety of weapons including: machine guns, shotguns, pistols, sniper rifles, alien energy guns, explosive weapons, grenades, energy swords, bare fists and a knife. (the energy sword is nothing to worry about, when a player gets hit by it they simply die as they would by any weapon (when a player dies they just look like they are unconscious) the knife is only used when a player sneaks up behind another player and holds the button they would normally use to swing there fist. they then perform an 'assassination'. in this maneuver they pull out the knife and stab the other player in the chest, neck, back or head. (its about as 'gorey' as watching 2 children throw fake punches at each other, and falling down pretending to die). the blood and gore aspect is not as bad as it sounds, when you kill someone/something blood will 'spatter' on the ground and/or walls. (since you are mostly fighting aliens, blood is typically blue, purple, yellow, green and red).
as for the 'gore' when you shoot someone/something, you do see minor liquids come out (blood).
other minor 'gore' includes, brief scenes and gameplay where a alien parasitic race, called 'the flood', attack there alien and human victims, and transform them into slow moving fungi and insect looking creatures. (its not vary graphic, other then male soldiers briefly screaming). (almost all of this goes the same for all the halo games)
all-in-all its a great game with 10 years of awards to show for it!
thank you for reading this is the first parental review i've written i hope it will be found useful.