This description sounds awfully familiar. I wonder why is that... Oh, I know! That's the description of The Tribe, a really cool 90's TV Show. 
 I'll tell you a little bit about the show... 
 There is a virus that wiped out all the adults. Not a single adult survived, only the childern and the teens are left in the world and no one knows what caused it. There is no electricity anymore, the food is scarce and the world is a dangerous place to live in. The tribes are forming, the battle over the dominance is constant, but they all have one common goal, to find the cure, because as they get older, they die too. 
 Really original, right? Mr. Chris Weitz, I think you should stick with the Movie Industry and leave writing to people who don't need to steal other people works to write books.