、。
She said she was twenty, but it was a lie.
Literal
She [topic-は] twenty-years-old [is-だ] [quotative-と] said [but-が], that [contrast-は] lie was.
Two clauses linked by clause-final が ('but'). The first reports her claim via quotative と; the second contradicts it. それは picks up the previous statement and contrasts it with 嘘 ('lie'). 嘘 is the noun for a lie, with the standard verbal collocation 嘘をつく ('tell a lie').