Sum up:
Left 4 Dead is from the makers of Half-life 2, and it's basically a first-person shooter, involving men and zombies duking it out with a variety of weapons. The main draw is multi-player (both split-screen and online), where you either take the role of the "Survivors," a group of four humans who survived the mysterious "disease"; or the "Infected," a group zombies. You then pick a map, and play until the other group is wiped out.

Recommended age:
The ESRB rated this game "M" for Mature, listing "Blood and Gore," "Intense Violence" and "Language." The "Blood and Gore" obviously refer to the blood that splatters and spurts in heavy amounts whenever enemies hit/shoot each other. It also refers to the grotesque appearance and abilities of enemy zombies. An example being the "Boomer," who can spew vomit on enemy Survivors (blinding them and attracting other zombies), and then blow up, leaving blood splatter all over their enemy.

The "Intense Violence" refers to the characters blowing each other apart with a variety of weapons like shotguns, assault rifles, pistols, etc., and the fact that many of the zombies can tear at enemies with their appendages. The "Language" will consist of words like d*mn, hell, a**, bastard, b*tch, etc. And depending who you play with online, there could be plenty of uses of the f-word and sh*t. So, as you've read, this game is obviously suited to an adult audience, and as such, I recommend this game for ages 17+.