A senior police official said they were questioning the teenager as they suspect that he was involved in the incident which took place in Gautampuri on Friday evening.
Delhi Police is yet to arrest the person who slit the girl's throat and inflicted other injuries on her two days ago.
Police had earlier detained 22 people in connection with the incident but no one involved in the incident has been arrested. "We are investigating the matter," a senior police official said.
Doctors at the AIIMS Trauma Centre, where she is being treated, said the victim was stable and having semi-solid food.