How is Supergirl older than Superman?
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Two space pods leave the station, one going 200 auph (autronomical units per hour), the other going 100 auph. The first one has a baby boy, less than a year old in suspended animation, no aging. The other pod has a 12 year old girl, no aging. Even though they left at the same time, they did not travel the same speed, and in the comics, didnt even use the same path.
When the first pod arrives, the baby begins to age. Twenty years later, the baby is now 20, while the 12 year old girl has just now arrived and began to age.
That is how Superman, 20, is now older than Supergirl, 12.
In the comics, I am familiar with, Kara first went into the sun, her pod was programmed to put her in the sun to gain power, then go to Earth. Also, in the new CW TV Show, Kal-el had a half brother, that also made the journey, while the New 52, Supermans father sent a test ship to earth as well, with a pilot that Supergirl would eventually fall in love with. Each of these pods also arrived at different times, travelling different speeds, and perhaps different paths, making the ages of the occupants vary.