Despite being more youthful in appearance than her cousin, Superman, Supergirl is actually older than him. Though it hasn't always been the case, most versions of the two have interpreted Kara as being chronologically older than Clark, while being biologically younger. The unique ties they share shines a light on how such a shift has affected the two's family dynamic.
Supergirl has been a part of Superman's cast since she first appeared in Action Comics #252 in a story by Otto Binder and Al Plastino. Emerging from a capsule, Supergirl told the harrowing story of her escape from Argo City, one of Krypton's last remaining civilizations. After witnessing a recording of Superman and discovering he was from Krypton, Kara's parents sent her to Earth to learn from the Man of Steel, unaware the hero is actually Kara's cousin.
Related: Superman's New Origin Changes The Real Reason He Escaped Krypton
While the Silver Age story established Supergirl as being born after Krypton's destruction, more modern takes on Kara have presented her as a teenager at the planet's pivotal moment. In Superman/Batman #9 by Jeph Loeb and Michael Turner, Kara discusses her father's attempt to save her with a ship similar to her cousin's. However, Kara's ship was caught in a meteor made of Kryptonite, which she believes kept her in a state of suspended animation for decades. By the time the ship was freed and could relocate to Earth, Kara was virtually the same age, while her baby cousin had become a fully grown adult. A similar, but altered, origin was shown in the New 52, where Kara was intentionally placed in a stasis pod by her father, which orbited the solar system before sending her to Earth.
So despite the Silver Age outlier, it's accepted that
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