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[Adventure Time] Why does Farmworld Marceline look so old?

Yes, I know that regular Marceline is 1000 years old, but she looks 17. Her Farmworld counterpart looks her age.
gpt
Marceline was born a half-human, half-demon. In the main universe, when the Bomb landed and the world ended in the Mushroom War, she helps the remaining humans escape to another landmass whilst fighting off the vampires and mutants that prey on them. 
  
In the main universe, she ends up getting bitten by one of the vampires, making her now part vampire. This freezes her physiology at the age she was at the time.
  
In the Farmworld, the world doesn't end in the Mushroom War but instead by getting frozen over by the crown. Because of this, creatures such as the vampires (presumably) never arise to become a menace, and Marceline, isolated in that cave for a thousand years, never becomes a vampire.
  
But because she's still half-demon, she can survive that long. She just ages as well.