Lost Boy / Christina Henry.

On an island lives a boy called Peter and his band of merry lost boys, young forever. That is, unless they get sick or killed by pirates or eaten by crocodiles or unless--inexplicably--they grow old. For some of them do grow old, and nobody knows why. One of these boys is called Jamie, and he was the first boy that Peter ever brought to the island. Jamie's lived there for longer than he can remember, and it's not all fun and games. Peter thinks the boys are replaceable, that if one dies or grows up, he can swap in another from the Other Place, the place where he collects new playmates. Jamie looks out for the boys and takes care of them. He does everything Peter does not. Jamie tries to keep them alive because, to him, they matter. When Peter steals a boy who is too young, Jamie takes the boy under his wing. But Peter won't have that, for nobody will ever take Jamie from him. Ever.