Price, Lissa. 
STARTERS
New York : Delacorte, 2012
IL YA
ISBN 0385742371

The Spore Wars have killed everyone between the ages of 20 and 60.  Callie is left trying to care for her sick younger brother, but while they struggle to survive, wealthy senior, or "Enders" and their grandchildren continue their lavish lifestyles.  Desperate for money, Callie signs up with a secret business that allows Enders to "rent" the bodies of teens and experience youth again.  When Callie wakes up in the middle of a session, she's immersed in her renter's life ... and soon find out their plans are decidedly darker than she signed up for.  (Isinglass Teen Award Committee, 2015)