Updated November 10th: read my full review of Fallout 3 here

Sum up:
Fallout 3 is an action RPG, and takes place 200 years after a nuclear explosion that destroyed most of the world. All survivors now live in bunkers, and very few of them ever wander out onto the still radioactive surface. The main character's father does however, and he then sets out to find him...

This game is from the makers of The Elder Scrolls: Oblivion, and features much of the same gameplay, with a huge explorable overworld (in the form of a ruined Washington D.C.), entirely non-linear quests (you don't even have to follow the main story), and every one of your choices affect the game's world and characters. The game also has a very realistic weapon system, where the weapons break down over time; a fully customizable ability and armor system; hundereds of hours of possible gameplay; and a companion in the form of a dog named "Dogmeat," who'll fight alongside you.

Recommended age:
The ESRB rated this game "M" for Mature, listing "Blood and Gore," "Intense Violence," "Sexual Themes," "Strong Language," and "Use of Drugs." The "Blood and Gore" will consist of heavy amounts of blood spurting and splattering on walls, floor, clothing, you name it, when you shoot/kill enemies; the enemies consist of a variety of grotesque and mutated (thanks to the radiation) human/monsters, giant cockroaches, etc; and you'll be able to blow off enemy heads and limbs, resulting in heavily gory detail (there's no word on whether you can disable blood or gore).

The "Intense Violence" refers to the game's extreme combat and damage system, where the physics system accounts for every limb you shoot. This means if you shoot an arm with a powerful weapon, it'll fall limp at the enemy's side, and in turn not allow him to use it. If you shoot his leg, it'll crush under the weight and he'll have to hop on one limb, etc. And your character is equipped with dozens of violent and powerful weapons like shotguns, rifles, rocket launchers, and even a mini nuclear bomb launcher.

The "Strong Language" will most definitely consist of words like, hell, d*mn, bastard, b*tch, plenty uses of the the f-word, and the word sh*t. And "Use of Drugs" refer to the game's item/power-up system, where you'll be able to use a variety of prescription drugs to counteract the radiation and enhance your powers. So as you've read, this game isn't nearly appropriate for a younger audience, and I recommend it for ages 17+.