This is addressed in the film's original script (back when the film was called I'm.Mortal) which offers a number of 'info-dumps' about the clocks that don't seem to have made it into the final cut of the film.
All of humanity has been genetically engineered.
In the not-too-distant future the aging gene has been switched off.
(NB: The minor variation in the biological age of adults is due to the
differing time of life an individual matures. Full maturation triggers
the aging gene to switch off and the "body clock" to begin ticking.)
"Time" can be moved from person to person.
Time is transferred from person to person by way of a unique
handshake. The right wrists are held together - the time conveyed by
the electricity in the pulse. The one who has their hand on top of the
other - who literally has the "upper hand" - is the one extracting the
time. Both parties monitor the amount of time exchanged by watching
the body clock on their left wrist.
Running out of time causes a heart attack.
In this world, the genetically rewired circuitry of the body that
keeps you young is also what kills you. Suddenly Rachel suffers an
irreversible shock to the system - a massive heart attack. She is
hurled off her feet, dead before she hits the ground.
The clocks are biological, not technological.
Greta proudly shows off her newborn's wrist with its tiny but
perfectly-formed unticking clock.
GRETA There it is.
We FOCUS on the digits indicating years - "1".
BOREL: Shame we've got to wait twenty years - we could use that year right now.
GRETA: It's a miracle, isn't it - how our bodies know when to switch it on? Nature sure is beautiful.
BOREL: Except it's not nature. We don't even know any more how our bodies do it. Why a year? Why are we even born with this clock?