I don't remember the exact details of the book, but I read it around 2008/9, and believe it was quite new.
I don't remember the title, but I was under the impression it was called Hunted or something (although Wikipedia and Google searches have left me with nothing).
The book involved people who took care of children (I believe it was usually one child to a carer) and the children received some sort of medical treatment to keep them young. However the general population didn't have kids (due to being infertile I think) and rent these kids for a day.
I don't really remember how the plot progresses but I think some of the kids
Escape from their carers and try to shut down the system.
The book cover had a triangular street sign on it, in a dark road with only the street sign and a little bit around it lit up, and on the sign was a child and possibly an adult or another child.
I believe the protagonist (who was one of the people stuck in a child's body) was over a century old.