Updated April 30th: read my full review of Gears of War 2 here

Sum up:
Gears of War 2 is a third-person action shooter, and is basically a bigger and badder version of the last. There's never been much to the game's story (or gameplay; you just take cover and shoot, shoot, blow up) so I'll just list some of the game's enhancements. The Unreal graphics engine has been beefed up (looks better); the game's cutscenes will feature a more realistic facial engine for characters; you'll now be able to use downed enemies as shields; grenades can be used as wall mines; and you can now drop in and out of the co-op mode at anytime, including the ability to have both players pick a different difficulty and experience it at the same time.

Recommended age:
The ESRB rated this game "M" for Mature, listing "Blood and Gore," "Intense Violence" and "Strong Language." The "Blood and Gore" refers to the riculous amount of thick blood splatter whenever you hit, saw in half, or shoot an enemy, and gibbing (enemies blow into large chunks) when you blow them up. However, like the last game, there will be an "Extreme Content" filter, which will replace all blood and gibbing (in gameplay, not cutscenes) with sparks.

The "Strong Language" refers to the game's use of words like d*mn, hell, bastard, b*tch, a**, a**hole, bullsh*t, sh*t (there where 58 uses of this word in the last game), and a few uses of the f-word. However, if you enable the "Extreme Content" option, it'll disable all f-words (but to be clear, only f-words).

The "Intense Violence" refers to the game's scary, tense and almost horror-like atmosphere/enemies, and the fact that you're blowing apart bizarre enemies with very destructive weapons. So, as you've read, there is no way this is appropriate for younger audiences, even with the extreme content off, and I'd recommend this game for ages 17+. If you want more detail on what this game will contain, read my first Gears of War content review here.