Silena's age is never revealed in the Percy Jackson book series.
She is played by 19-year-old Zoe Aggeliki in the film Percy Jackson and the Lightning Thief.
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Her name is Silena Beauregard.
No she is not mentioned in The Lighting Theif
They started going out in the Demigod Files, specially the story Percy Jackson and the Bronze Dragon.
Its when silena pretends to be Clarrise to slave the huge drakon but Percy knew it was not Clarrise so silens was killed by the drakon.
It is Silena Boureguard, near the end of the Last Olympian, right before she dies she holds up her wrist and she holds up her charm bracelet with the picture of the scythe. Though she was a spy, she sacrificed her life to make things right.