The 1967 book was much better than the 1976 movie, though (like much sf) its future chronology has been overtaken by real time. The introduction sets the backstory:
"The seeds of the Little War were planted in a restless summer during the mid-1960s, with sit-ins and student demonstrations as youth tested its strength. By the early 1970s over 75 percent of the people living on Earth were under 21 years of age. The population continued to climb — and with it the youth percentage. In the 1980s the figure was 79.7 percent.
In the 1990s, 82.4 percent.
In the year 2000 — critical mass."
In this future the population is monitored by a flower-shaped crystal embedded in one’s palm at birth. From birth to age seven it’s yellow, then from seven to fourteen it’s blue, and from fourteen until twenty-one it’s red. On one’s twenty-first birthday (“Lastday”) the crystal begins to alternate between red and black as a sign to report to a Sleepshop for euthanasia. As Lastday ends the crystal turns completely black and those who have not reported in are now legal prey of the Sandmen, a cadre of armed enforcers. Logan 3 is a Sandman who decides to infiltrate and destroy the Sanctuary he’s heard Runners whisper about. But he comes to question his training and becomes a Runner himself.