The girl weighed only 20 kilos (44 pounds), and said she had been fed only bread and water in her captivity, BBC reported.
Her only company had been that of a dog and a monkey, and she said she was beaten with a belt if she tried to eat the leftover food thrown to the pets.
Her carers have been arrested and charged with slavery and abuse.
The teenager was found in the Argentine capital, Buenos Aires, by one of her biological sisters, who had lost track of her.
The girl, who has been hospitalized, said she had been out of the garage only twice in nine years.
According to officials, her foster parents took the girl into care provisionally in 2001 after a court found that her biological parents, who had seven more children, were financially unable to look after her.
At first, the two families kept in contact, but it is unclear what happened after 2005 and why her biological family reportedly lost track of the girl.
Her foster parents were waiting for the girl’s adoption papers to be finalized.