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Accelerated aging is a common side effect of the cloning process as developed by LuthorCorp.

Julian Luthor's clones.

Julian Luthor's clones.

Known victims

LuthorCorp clones
Victims of Chrissy Parker
Metahumans
Cursed by magic

Season Two

The process was for aging clones to adulthood was first developed under the supervision of Lionel Luthor, by the LuthorCorp geneticist Pete Dinsmore. Dinsmore first applied tested the process on a series of infant rabbits, which he was successful in aging to adulthood. With this success, Dinsmore attempted to clone his deceased daughter Emily, and then applied the experimental process to accelerate her aging so that she would be approximately the same age as the original Emily when she died. The acceleration process involved the introduction of Kryptonite into her biology.

However, the aging process resulted in severe side effects. Specifically, the Emily's moral center was damaged beyond repair, which resulted in her becoming a childhood sociopath with no internal concept of right and wrong. She also gained the ability to move at super speed.

Season Three

Lionel later abandoned the project after Emily murdered her father, and it was discovered that Emily's accelerated aging would not slow down.

References