Outland Exile: Book 1 of Old men and Infidels

The world is getting older—and younger.
The United States is dead and the Unity killed it.
After catastrophic wars, The Unity rules from its citadel, leaving the outlands to savages and its altered plants. Providing full employment, and rec drugs at quite reasonable prices, the Unity mandates retirement at forty. Just before you disappear. 17-year-old Lieutenant Malila Chiu, is a veteran officer who finds her career in tatters. Malila’s one option is to enter the outlands to fix her mistakes.
Dropping from fatigue, she wakes to find a hideous, ancient savage, Jesse Johnstone, has murdered her men and holds a knife at her throat.