Wither by Lauren DeStefano 

By age sixteen, Rhine Ellery has four years left to live. She can thank modern science for this genetic time bomb. A botched effort to create a perfect race has left all males with a lifespan of 25 years, and females with a lifespan of 20 years. Geneticists are seeking a miracle antidote to restore the human race, desperate orphans crowd the population, crime and poverty have skyrocketed, and young girls are being kidnapped and sold as polygamous brides to bear more children…. Read more.