The Immortality Virus by Christine Amsden

"In the mid-21st century, the human race stopped aging. Those who know why aren't talking, and the few who are brave enough to ask questions tend to disappear. To an elite few, The Change means long life and health, but to the increasing masses, it means starvation, desperation, and violence.

Four centuries after The Change, Grace Harper, a blacklisted P.I., sets off on a mission to find the man responsible for it all and solicit his help to undo The Change - if he's still alive. To complicate matters, Grace's employer is suspected of murdering his father, and when the police learn of their connection, they give her a choice - help them find the evidence they need to convict Matthew Stanton, or die. But if they discover Grace's true mission, they won't hesitate to kill her in order to protect their shot at immortality." (Barnes and Noble).