Shade's Children by Garth Nix.
Here is the description from Amazon:
In a futuristic urban wasteland, evil Overlords have decreed that no child shall live a day past his fourteenth birthday. On that Sad Birthday, the child is the object of an obscene harvest resulting in the construction of a machinelike creature whose sole purpose is to kill.
The mysterious Shade -- once a man, but now more like the machines he fights -- recruits the few children fortunate enough to escape. With luck, cunning, and skill, four of Shade's children come closer than any to discovering the source of the Overlords' power -- and the key to their downfall. But the closer the children get, the more ruthless Shade seems to become ...
--I got this from my local library. This is the first in a series.