Dramatic footage shows them scaling the side of the building and helping the parents escape their third-floor flat on Sunday afternoon.
Mattresses were brought to the foot of the building and the couple's six-month-old baby was thrown to safety.
She was taken to hospital in a critical condition but is said to be recovering.
One of the youths involved in the rescue, Tahmid, told the Ouest-France website that the decision to throw the infant from the balcony was "the last possible solution" as smoke billowed from the building in the Bottière district of Nantes.
