YA novel about dystopian future with non-human overlords and teenage heroes

I read a YA novel in the early 2000s that I think takes place on Earth some time in the future. The human race has been enslaved by some other race, possibly aliens. If I remember correctly, humans were kept as slaves for some purpose and then exterminated at age 16, and I think some girls were kept to breed.

I remember scenes of a band of teenagers walking around sewers fighting these aliens who bled a blue "ichor." The only thing I distinctly remember about the book was that it was the first place I saw the word "ichor."

This is a pretty sorry description; but does anyone have a clue what this book was?