A teenager who with her twin sister robbed an octogenarian they met on an Internet dating site was sentenced on Thursday to five years in prison.

The woman, Shaina Foster, 18, had pleaded guilty to one count of robbery in exchange for the five-year sentence in a deal with the Manhattan district attorney’s office. She and her sister, Shalaine Foster, admitted that they tied up Paul Aronson, 84, in his house and took his cash and credit cards after he had taken them both on a dinner date.

Citing the seriousness of the crime, Justice Michael Obus of State Supreme Court in Manhattan said he would not grant Ms. Foster youthful offender status, which would have made her eligible for a lesser sentence.

Mr. Aronson lay helpless on his living room floor, bound with zip ties, unable to eat or drink, from 8:15 p.m. on Oct. 1 until the next morning, when a neighbor discovered him.