The lifeclock is the crystal in the palm of every citizen's right hand.
In The Novel, a person's age is revealed by their palm flower crystal embedded in the palm of their left hand that changes color every seven years, yellow (age 0-6), then blue (age 7-13), then red (age 14-20), then blinks red and black on Lastday, and finally turns black at 21.
In The Movie, the crystal is embedded in the right hand. The color scheme is also different: clear from birth till age 8, yellow from ages 8 to 16, green from 16 to 24, and red from 24 to 30. Citizens typically wear clothing that is the same color as their crystal.
As Lastday approaches, the crystal will begin blinking from red to black.
Lifeclocks only function inside the city. Anyone who ventures outside will find their lifeclock has once again turned clear.